dix: Remove extension aliases

This appears to be essentially unused. The only known client-side
library for the SELinux extension is xcb, which does not look for the
name "Flask". The "SGI-GLX" alias for GLX appears to be a bit of
superstition at this point, NVIDIA's driver does not expose it and Mesa
does not check for it.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Keith Packard <keithp@keithp.com>