Fix implicitly registered converter warning

[TomAugspurger]

This shouldn't warn, since it's a Series.plot.

The warning should only show up for Axes.plot(datetimelike, ndarray).

In [1]: import pandas as pd

In [2]: import numpy as np

In [3]:    ts = pd.Series(np.random.randn(1000),
   ...:                   index=pd.date_range('1/1/2000', periods=1000))
   ...:    ts = ts.cumsum()

In [4]: ts.plot()
/Users/taugspurger/sandbox/pandas/pandas/plotting/_matplotlib/converter.py:96: FutureWarning: Using an implicitly registered datetime converter for a matplotlib plotting method. The converter was registered by pandas on import. Future versions of pandas will require you to explicitly register
matplotlib converters.