brd: remove unused brd_mutex

Remove unused mutex brd_mutex. It is unused since the commit ff26956875c2
("brd: remove support for BLKFLSBUF").

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/brd.c

@@ -60,7 +60,6 @@ struct brd_device {
 /*
  * Look up and return a brd's page for a given sector.
  */
-static DEFINE_MUTEX(brd_mutex);
 static struct page *brd_lookup_page(struct brd_device *brd, sector_t sector)
 {
 	pgoff_t idx;