bonding: remove no longer relevant vlan warnings

These warnings are no longer relevant. Even when last slave is
removed, there is a valid address assigned to bond (random).
The correct functionality of vlans is ensured by maintaining unicast
list in vlan_sync_address().

Suggested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>