net_sched: remove unused htb drop_list

After commit a09ceb0e ("sched: remove qdisc->drop"),
it is no longer used.

Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>