DEPR: Fixed warning for implicit registration
Reminder: We want people to explicitly call `pandas.plotting.register_matplotlib_converters()` when using a matplotlib plotting method with a pandas object. We implicitly call it as part of Series/Frame.plot.


```python
In [1]: import pandas as pd
In [2]: import matplotlib.pyplot as plt
In [3]: fig, ax = plt.subplots()
In [4]: s = pd.Series(range(12), index=pd.date_range('2017', periods=12))
In [5]: ax.plot(s)
## -- End pasted text --
Using matplotlib backend: TkAgg
/Users/taugspurger/sandbox/pandas/pandas/plotting/_converter.py:129: 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.

To register the converters:
        >>> from pandas.plotting import register_matplotlib_converters
        >>> register_matplotlib_converters()
  warnings.warn(msg, FutureWarning)
Out[1]: [<matplotlib.lines.Line2D at 0x1077d7b38>]
```

I'm not quite sure what happened in 
https://github.com/pandas-dev/pandas/pull/18307, but the warning wasn't showing up by default.

Closes https://github.com/pandas-dev/pandas/issues/24963