BUG: plotting regular tz-aware timeseries gives UserWarning 
With this example of plotting a tz-aware, regular timeseries:

```
df = pd.DataFrame(np.random.randn(100, 3), index=pd.date_range("2012", freq='H', periods=100, tz='UTC'), columns=['a', 'b', 'c'])
df.plot()
```
gives a warning that the user shouldn't see I think (there is nothing to do about it):

```
/home/joris/scipy/pandas/pandas/core/arrays/datetimes.py:1099: UserWarning: Converting
to PeriodArray/Index representation will drop timezone information.
  UserWarning,
```

When setting a filterwarning to error, you can see the warning is coming from a `to_period()` call in the matplotlib plotting backend included in pandas:

```
In [5]: warnings.simplefilter("error", UserWarning)

In [6]: df.plot()  
---------------------------------------------------------------------------
UserWarning                               Traceback (most recent call last)
<ipython-input-6-848b80e64df8> in <module>
----> 1 df.plot()

~/scipy/pandas/pandas/plotting/_core.py in __call__(self, *args, **kwargs)
    846                     data.columns = label_name
    847 
--> 848         return plot_backend.plot(data, kind=kind, **kwargs)
    849 
    850     def line(self, x=None, y=None, **kwargs):

~/scipy/pandas/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

~/scipy/pandas/pandas/plotting/_matplotlib/core.py in generate(self)
    261         self._compute_plot_data()
    262         self._setup_subplots()
--> 263         self._make_plot()
    264         self._add_table()
    265         self._make_legend()

~/scipy/pandas/pandas/plotting/_matplotlib/core.py in _make_plot(self)
   1050             from pandas.plotting._matplotlib.timeseries import _maybe_convert_index
   1051 
-> 1052             data = _maybe_convert_index(self._get_ax(0), self.data)
   1053 
   1054             x = data.index  # dummy, not used

~/scipy/pandas/pandas/plotting/_matplotlib/timeseries.py in _maybe_convert_index(ax, data)
    252 
    253         if isinstance(data.index, ABCDatetimeIndex):
--> 254             data = data.to_period(freq=freq)
    255         elif isinstance(data.index, ABCPeriodIndex):
    256             data.index = data.index.asfreq(freq=freq)

~/scipy/pandas/pandas/core/frame.py in to_period(self, freq, axis, copy)
   8379         axis = self._get_axis_number(axis)
   8380         if axis == 0:
-> 8381             new_data.set_axis(1, self.index.to_period(freq=freq))
   8382         elif axis == 1:
   8383             new_data.set_axis(0, self.columns.to_period(freq=freq))

~/scipy/pandas/pandas/core/accessor.py in f(self, *args, **kwargs)
     97         def _create_delegator_method(name):
     98             def f(self, *args, **kwargs):
---> 99                 return self._delegate_method(name, *args, **kwargs)
    100 
    101             f.__name__ = name

~/scipy/pandas/pandas/core/indexes/datetimelike.py in _delegate_method(self, name, *args, **kwargs)
    986 
    987     def _delegate_method(self, name, *args, **kwargs):
--> 988         result = operator.methodcaller(name, *args, **kwargs)(self._data)
    989         if name not in self._raw_methods:
    990             result = Index(result, name=self.name)

~/scipy/pandas/pandas/core/arrays/datetimes.py in to_period(self, freq)
   1097                 "Converting to PeriodArray/Index representation "
   1098                 "will drop timezone information.",
-> 1099                 UserWarning,
   1100             )
   1101 

UserWarning: Converting to PeriodArray/Index representation will drop timezone information.
```