glthread: remove _mesa_post_marshal_hook, because it's not very useful

and also remove the useless forward declaration of enum marshal_dispatch_cmd_id.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4124>

src/mesa/main/glthread.h

@@ -48,7 +48,6 @@
 #include "util/u_queue.h"
 #include "GL/gl.h"
 
-enum marshal_dispatch_cmd_id;
 struct gl_context;
 struct _mesa_HashTable;