[VLAN]: Remove non-implemented ioctls

The GET_VLAN_INGRESS_PRIORITY_CMD/GET_VLAN_EGRESS_PRIORITY_CMD ioctls are
not implemented and won't be, new functionality will be added to the netlink
interface. Remove the code and make the ioctl handler return -EOPNOTSUPP
for unknown commands instead of -EINVAL.

Also remove a comment about passing unknown commands to the underlying
device, that doesn't make any sense since its a VLAN specific ioctl and
if its not implemented here, its implemented nowhere.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>