mesa/x11: Remove the swrast-classic-based fake libGL

If you want this you will almost certainly be happier with the gallium
version, giving you llvmpipe instead of swrast-classic.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10153>

meson_options.txt

@@ -290,7 +290,7 @@ option(
   'glx',
   type : 'combo',
   value : 'auto',
-  choices : ['auto', 'disabled', 'dri', 'xlib', 'gallium-xlib'],
+  choices : ['auto', 'disabled', 'dri', 'xlib'],
   description : 'Build support for GLX platform'
 )
 option(