meson: drop gallium-media argument

This argument is the wrong approach for handling gallium media state
trackers, since it doesn't allow for an auto option. Instead we'll use
tristates, which do allow for auto.

This option has never been wired to anything anyway.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

meson_options.txt

@@ -49,12 +49,6 @@ option(
   value : 'auto',
   description : 'comma separated list of gallium drivers to build. If this is set to auto all drivers applicable to the target OS/architecture will be built'
 )
-option(
-  'gallium-media',
-  type : 'string',
-  value : '',
-  description : 'comma separated list of gallium media APIs to build (omx,va,vdpau,xvmc).'
-)
 option(
   'gallium-extra-hud',
   type : 'boolean',