bonding: delete unused arp_mon_pt

Now all received packets are handled by bond_handle_frame,
and arp_mon_pt isn't used any more.

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>