xf86-video-mga: remove the GlxSetVisualConfigs stub and friends

The function was an empty since 2008 at least. Remove it alongside the
unused GLX visuals code.

With this all the GL/GLX dependencies in the driver are gone.

Cc: Mathieu Larouche <mathieu.larouche@matrox.com>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
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There's a GLX reference in mga_ucode.h - a file which was seemingly
never used since day 1. I'm not going to bother with that, since I've
taken enough of a diversion already ;-)