bonding: delete unused ad_timer

Now we use agg_select_timer and ad_work.

Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Weiping Pan <panweiping3@gmail.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>