MAINTAINERS: remove unused radeon drm pattern

Commit 8dcedd7e87f4 ("UAPI: (Scripted) Disintegrate include/drm") moved
the file, remove the pattern.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>