blk-mq: remove unused function blk_mq_queue_stopped()

blk_mq_queue_stopped() doesn't have any caller, which was found by
code coverage test, thus remove it.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20220818063555.3741222-1-yukuai1@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>