xf86-video-tdfx: 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: Guillem Jover <guillem@hadrons.org>
Cc: Adam Jackson <ajax@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>