net_sched: fix a missing idr_remove() in u32_delete_key()

When we delete a u32 key via u32_delete_key(), we forget to
call idr_remove() to remove its handle from IDR.

Fixes: e7614370

 ("net_sched: use idr to allocate u32 filter handles")
Reported-by: Marcin Kabiesz <admin@hostcenter.eu>
Tested-by: Marcin Kabiesz <admin@hostcenter.eu>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>