blk-throttle: cleanup tg_update_disptime()

blk-throttle: cleanup tg_update_disptime()

tg_update_disptime() only need to adjust postion for 'tg' in
'parent_sq', there is no need to call throtl_enqueue/dequeue_tg(),
since they will set/clear flag THROTL_TG_PENDING and increase/decrease
nr_pending, which is useless. By the way, clear the flag/decrease
nr_pending while there are still throttled bios is not good for debugging.

There are no functional changes.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220827101637.1775111-4-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>