VIS: register_matplotlib_converters is exposed through plotting backend
With the recent plotting backend refactor, the `pandas.plotting.register_matplotlib_converters` is now referring to the "general" `register` which calls the plotting backend, instead of just being the matplotlib register function.

It probably doesn't make much sense to expose this through the plotting backend? 
It is maybe done for consistency (everything goes through the plotting backend, also internally), but personally I found this indirection confusing when looking at the source code (for this case).

cc @datapythonista 

