bonding: remove bond_vlan_used()

We're using it currently to verify if we have vlans before getting the tag
from the skb we're about to send. It's useless because the vlan_get_tag()
verifies if the skb has the tag (and returns an error if not), and we can
receive tagged skbs only if we *already* have vlans.

Plus, the current RCUed implementation is kind of useless anyway - the we
can remove the last vlan in the moment we return from the function.

So remove the only usage of it and the whole function.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>