blk-mq: remove useless checking in queue_requests_store()

blk_mq_queue_attr_visible() already checked queue_is_mq(), no need to
check this again in queue_requests_store().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>