block: move sched debugfs register into elvevator_register_queue

block: move sched debugfs register into elvevator_register_queue

sched debugfs shares same lifetime with scheduler's kobject, and same
lock(elevator lock), so move sched debugfs register/unregister into
elevator_register_queue() and elevator_unregister_queue().

Then we needn't blk_mq_debugfs_register() for us to register sched
debugfs any more.

Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-7-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>