blk-throttle: remove incorrect comment for tg_last_low_overflow_time

Function tg_last_low_overflow_time is called with intermediate node as
following:
throtl_hierarchy_can_downgrade
  throtl_tg_can_downgrade
    tg_last_low_overflow_time

throtl_hierarchy_can_upgrade
  throtl_tg_can_upgrade
    tg_last_low_overflow_time

throtl_hierarchy_can_downgrade/throtl_hierarchy_can_upgrade will traverse
from leaf node to sub-root node and pass traversed intermediate node
to tg_last_low_overflow_time.

No such limit could be found from context and implementation of
tg_last_low_overflow_time, so remove this limit in comment.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
Link: https://lore.kernel.org/r/20221205115709.251489-8-shikemeng@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -1762,7 +1762,6 @@ static unsigned long __tg_last_low_overflow_time(struct throtl_grp *tg)
 	return min(rtime, wtime);
 }
 
-/* tg should not be an intermediate node */
 static unsigned long tg_last_low_overflow_time(struct throtl_grp *tg)
 {
 	struct throtl_service_queue *parent_sq;