null_blk: remove set but not used variable 'q'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/block/null_blk_main.c: In function 'end_cmd':
drivers/block/null_blk_main.c:609:24: warning:
 variable 'q' set but not used [-Wunused-but-set-variable]

It not used any more after commit
e50b1e32 ("null_blk: remove legacy IO path")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -606,12 +606,8 @@ static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq, int can_wait)
 
 static void end_cmd(struct nullb_cmd *cmd)
 {
-	struct request_queue *q = NULL;
 	int queue_mode = cmd->nq->dev->queue_mode;
 
-	if (cmd->rq)
-		q = cmd->rq->q;
-
 	switch (queue_mode)  {
 	case NULL_Q_MQ:
 		blk_mq_end_request(cmd->rq, cmd->error);