net/sched: sch_cbq: Delete unused delay_timer

delay_timer has been unused since commit c3498d34 ("cbq: remove
TCA_CBQ_OVL_STRATEGY support").  Delete it.

Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>