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>