MAINTAINERS: remove unused nbd.h pattern

Commit 13e71d69 ("nbd: Remove kernel internal header") deleted the
file, remove the pattern.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>