docs: remove documentation of MESA_PRIVATE_CMAP

This is used in the Mesa GLUT implementation, which has been separated
out into its own repo, and is largely abandoned in favour of FreeGLUT.

So let's not try to document MESA_PRIVATE_CMAP, as it's just going to
confuse users.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10281>
1 file changed