Remove deprecated arguments from plot_pair()

[agustinaarroyuelo]

plot_kwargs is an argument of plot_pair() yet it is not described in the documentation for this function.

To Reproduce
Go to the link and search for the description of plot_kwargs

Expected behavior
A complete docstring were plot_kwargs usage and behaviour is described.

[OriolAbril]

It might also be possible it is not even used at all, in which case it should be removed instead of documented

[agustinaarroyuelo]

You are right, we planned to deprecate plot_kwargs (see here) Maybe we can move forward with it instead of documenting

[OriolAbril]

Let's remove it then and update the issue to remove the deprecated arguments in plot_pair. I am also adding it to the v0.13 milestone.