BUG: DataFrame.plot(subplots=True) results in extra ticks
- [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.

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

---

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

```python
import pandas as pd
import numpy as np

df = pd.DataFrame(
    {
        'a': np.random.randint(100, size=100), 
        'b': np.random.randint(100, size=100), 
        'c': pd.date_range(start='1/1/2018', periods=100, freq='s'),
    }
)

axs = df.set_index(df['c'] + pd.to_timedelta(df['b'], unit='ms')).drop(columns='c').plot(subplots=True)
#[ax.minorticks_off() for ax in axs] # note uncomment this to work around issue

```

#### Problem description

When using `DataFrame.plot` with `subplots=True`, extra tick marks appear on the x axis.

![issue](https://user-images.githubusercontent.com/14239619/105246578-75114500-5b41-11eb-9b90-88446590266f.png)

#### Expected Output

No extra ticks. Uncomment the line above to get the below:

![workaround](https://user-images.githubusercontent.com/14239619/105246617-80fd0700-5b41-11eb-9c02-144e24566975.png)


#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 9d598a5e1eee26df95b3910e3f2934890d062caa
python           : 3.9.1.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.17763
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.2.1
numpy            : 1.19.5
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 49.6.0.post20210108
Cython           : None
pytest           : 6.2.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
