net/sched: Remove unnecessary WARNING condition for empty child qdisc in htb_activate

[ Upstream commit 2c2192e5 ]

The WARN_ON trigger based on !cl->leaf.q->q.qlen is unnecessary in
htb_activate. htb_dequeue_tree already accounts for that scenario.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: William Liu <will@willsroot.io>
Reviewed-by: Savino Dicanosa <savy@syst3mfailure.io>
Link: https://patch.msgid.link/20250819033632.579854-1-will@willsroot.io


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>