meson: remove deprecated osmesa-bits option

We've released several new major versions since this was deprecated in
202cab3d30 ("meson: deprecate specifying osmesa-bits"), so it's
probably about time to remove it.

Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20905>

meson_options.txt

@@ -392,14 +392,6 @@ option(
   value : false,
   description : 'Build OSmesa.'
 )
-option(
-  'osmesa-bits',
-  type : 'combo',
-  value : 'unspecified',
-  choices : ['unspecified', '8', '16', '32'],
-  description : 'DEPRECATED: Number of channel bits for OSMesa.'
-)
-
 option(
   'tools',
   type : 'array',