dm space map metadata: remove unused variable in brb_pop()

Remove the unused struct block_op pointer that was inadvertantly
introduced, via cut-and-paste of previous brb_op() code, as part of
commit 50dd842a.

(Cc'ing stable@ because commit 50dd842a did)

Fixes: 50dd842a ("dm space map metadata: fix ref counting bug when bootstrapping a new space map")
Reported-by: NDavid Binderman <dcb314@hotmail.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

drivers/md/persistent-data/dm-space-map-metadata.c

@@ -152,12 +152,9 @@ static int brb_peek(struct bop_ring_buffer *brb, struct block_op *result)
 
 static int brb_pop(struct bop_ring_buffer *brb)
 {
-	struct block_op *bop;
-
 	if (brb_empty(brb))
 		return -ENODATA;
 
-	bop = brb->bops + brb->begin;
 	brb->begin = brb_next(brb, brb->begin);
 
 	return 0;