null_blk: remove unused nullb device

The compiler rightfully complains:

drivers/block/null_blk_main.c: In function ‘null_complete_rq’:
drivers/block/null_blk_main.c:647:16: warning: unused variable ‘nullb’ [-Wunused-variable]
  struct nullb *nullb = rq->q->queuedata;
                ^~~~~

Fixes: 49f66136 ("nullb: remove leftover legacy request code")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -644,8 +644,6 @@ static void null_cmd_end_timer(struct nullb_cmd *cmd)
 
 static void null_complete_rq(struct request *rq)
 {
-	struct nullb *nullb = rq->q->queuedata;
-
 	end_cmd(blk_mq_rq_to_pdu(rq));
 }