MAINT: Remove doc/cython and doc/pyrex files.

Pyrex is obsolete and cython does a good job with Numpy these days.
These directories are not packages, so I don't think they need
deprecation.

Closes #4373.
DOC: Document removal of doc/pyrex and doc/cython directories.
Merge pull request #4418 from charris/remove-cython-pyrex-directories

MAINT: Remove doc/cython and doc/pyrex files.