MNT: deprecate the style module

[rcomer]

Rationale

#2323 and #2333 removed our internal use of this module.

This module does 2 things

• Merges dictionaries of styles taking account of a hard-coded subset of Matplotlib aliases (specifically not plural aliases like facecolors). If this was intended for general use I think it should handle all aliases, maybe making use of ArtistInspector.get_aliases so it could be artist specific. However a utility function for that probably belongs in Matplotlib (if anywhere) rather than here.
• Handles the Cartopy-specific facecolor="never", but there is some discussion of removing that completely at Make FeatureArtist a Collection #2323 (comment).

Implications

One less module to maintain.

[greglucas]

This module seems like it should have been a private helper and hopefully won't be used by others downstream... Although it does look like it came in fairly recently:
#1190

With your recent changes making FeatureArtist a Collection #2323 I think it is OK to get rid of this now, but it would be good to hear if @pelson or @QuLogic had any other things in mind for this module.

[QuLogic]

If we can avoid having to write our own stuff, that's probably for the best. I don't have any particular plans in mind for this module.