bonding: Remove obsolete source file 'bond_ipv6.c'

This file is now unused and should have been removed by commit
7c89943236750537d26421d9bbb6f6575e2d1e1b ("bonding, ipv4, ipv6, vlan:
Handle NETDEV_BONDING_FAILOVER like NETDEV_NOTIFY_PEERS").

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>