mesa: remove duplicate 'disabled extensions' list

mesa: remove duplicate 'disabled extensions' list

While parsing MESA_EXTENSION_OVERRIDE we keep track of the disabled
extensions, twice - in _mesa_extension_override_disables and
disabled_extensions.

Upon context creation, we use the former to modify the extensions list.
Yet, we still check the updated list against disabled_extensions.

Remove disabled_extensions, it's obsolete.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>