block: kill all_q_node in request_queue

all_q_node has not been used since commit 4b855ad3 ("blk-mq: Create
hctx for each present CPU"), so remove it.
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -548,7 +548,6 @@ struct request_queue {
 	struct rcu_head		rcu_head;
 	wait_queue_head_t	mq_freeze_wq;
 	struct percpu_ref	q_usage_counter;
-	struct list_head	all_q_node;
 
 	struct blk_mq_tag_set	*tag_set;
 	struct list_head	tag_set_list;