CLN: deprecate/remove the pandas.tseries.plotting.tsplot function
xref https://github.com/pandas-dev/pandas/pull/17814

As far as I can see, the `tsplot` function (https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/_timeseries.py#L23) is not used internally, and it is not exposed in the `pandas.plotting` namespace (it *is* still exposed in `pandas.tseries.plotting`). 
And I also don't think it is actually a useful function for users? (they would just do `ts.plot()`, and here you have to awkwardly provide an actual plotting function)