BUG: get UserWarning about FixedFormatter/FixedLocator in series.plot()
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

#### Code Sample, a copy-pastable example

```python
pd.Series({"20190102": 10, "20190103": 10}).plot()
```

#### Problem description

Got a UserWarning:
```
/home/.../miniconda3/lib/python3.8/site-packages/pandas/plotting/_matplotlib/core.py:1192: UserWarning: FixedFormatter should only be used together with FixedLocator
```

And to catch the Warning:
```python
import warnings
with warnings.catch_warnings():
    warnings.filterwarnings("error")
    pd.Series({"20190102": 10, "20190103": 10}).plot()
```
Got

```
---------------------------------------------------------------------------
UserWarning                               Traceback (most recent call last)
<ipython-input-5-4339a8efd56e> in <module>
      2 with warnings.catch_warnings():
      3     warnings.filterwarnings("error")
----> 4     pd.Series({"20190102": 10, "20190103": 10}).plot()

~/miniconda3/lib/python3.8/site-packages/pandas/plotting/_core.py in __call__(self, *args, **kwargs)
    845                     data.columns = label_name
    846 
--> 847         return plot_backend.plot(data, kind=kind, **kwargs)
    848 
    849     __call__.__doc__ = __doc__

~/miniconda3/lib/python3.8/site-packages/pandas/plotting/_matplotlib/__init__.py in plot(data, kind, **kwargs)
     59             kwargs["ax"] = getattr(ax, "left_ax", ax)
     60     plot_obj = PLOT_CLASSES[kind](data, **kwargs)
---> 61     plot_obj.generate()
     62     plot_obj.draw()
     63     return plot_obj.result

~/miniconda3/lib/python3.8/site-packages/pandas/plotting/_matplotlib/core.py in generate(self)
    268         for ax in self.axes:
    269             self._post_plot_logic_common(ax, self.data)
--> 270             self._post_plot_logic(ax, self.data)
    271 
    272     def _args_adjust(self):

~/miniconda3/lib/python3.8/site-packages/pandas/plotting/_matplotlib/core.py in _post_plot_logic(self, ax, data)
   1190             xticks = ax.get_xticks()
   1191             xticklabels = [get_label(x) for x in xticks]
-> 1192             ax.set_xticklabels(xticklabels)
   1193             ax.xaxis.set_major_locator(FixedLocator(xticks))
   1194 

~/miniconda3/lib/python3.8/site-packages/matplotlib/axes/_base.py in wrapper(self, *args, **kwargs)
     61 
     62         def wrapper(self, *args, **kwargs):
---> 63             return get_method(self)(*args, **kwargs)
     64 
     65         wrapper.__module__ = owner.__module__

~/miniconda3/lib/python3.8/site-packages/matplotlib/cbook/deprecation.py in wrapper(*args, **kwargs)
    449                 "parameter will become keyword-only %(removal)s.",
    450                 name=name, obj_type=f"parameter of {func.__name__}()")
--> 451         return func(*args, **kwargs)
    452 
    453     return wrapper

~/miniconda3/lib/python3.8/site-packages/matplotlib/axis.py in _set_ticklabels(self, labels, fontdict, minor, **kwargs)
   1791         if fontdict is not None:
   1792             kwargs.update(fontdict)
-> 1793         return self.set_ticklabels(labels, minor=minor, **kwargs)
   1794 
   1795     @cbook._make_keyword_only("3.2", "minor")

~/miniconda3/lib/python3.8/site-packages/matplotlib/axis.py in set_ticklabels(self, ticklabels, minor, **kwargs)
   1728             ticks = self.get_minor_ticks(len(locs))
   1729         else:
-> 1730             self.set_major_formatter(formatter)
   1731             locs = self.get_majorticklocs()
   1732             ticks = self.get_major_ticks(len(locs))

~/miniconda3/lib/python3.8/site-packages/matplotlib/axis.py in set_major_formatter(self, formatter)
   1589         formatter : `~matplotlib.ticker.Formatter`, ``str``, or function
   1590         """
-> 1591         self._set_formatter(formatter, self.major)
   1592 
   1593     def set_minor_formatter(self, formatter):

~/miniconda3/lib/python3.8/site-packages/matplotlib/axis.py in _set_formatter(self, formatter, level)
   1619                 and len(formatter.seq) > 0
   1620                 and not isinstance(level.locator, mticker.FixedLocator)):
-> 1621             cbook._warn_external('FixedFormatter should only be used together '
   1622                                  'with FixedLocator')
   1623 

~/miniconda3/lib/python3.8/site-packages/matplotlib/cbook/__init__.py in _warn_external(message, category)
   2088             break
   2089         frame = frame.f_back
-> 2090     warnings.warn(message, category, stacklevel)
   2091 
   2092 

UserWarning: FixedFormatter should only be used together with FixedLocator
```
#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.104-microsoft-standard
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.1
setuptools       : 49.3.1.post20200810
Cython           : None
pytest           : 6.0.1
hypothesis       : None
sphinx           : 3.2.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.2
html5lib         : None
pymysql          : 0.10.0
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.17.0
pandas_datareader: None
bs4              : 4.9.1
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.2
matplotlib       : 3.3.0
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.4
pandas_gbq       : None
pyarrow          : 1.0.0
pytables         : None
pytest           : 6.0.1
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.18
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
