MAINTAINERS: remove include/linux/ext3*

Moved to fs/ext3/ext3.h by commit 4613ad18 ("ext3: move headers to
fs/ext3/").

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>