remove _orig_color which is duplicate of _rgb

The GraphicsContextBase._orig_color member was introduced by https://github.com/matplotlib/matplotlib/commit/6cc8aaa3e2a493a8df72cc10a08c2422ecb37ae2 , but I don't believe it is necessary.  This patch removes that member and goes a small step further to put related special case in set_foreground.

This is a spin-off of https://github.com/matplotlib/matplotlib/issues/3393 where test coverage is a legit concern.  A default tests.py run on my linux box covers all paths touched by this code with the exception of set_greylevel.