vlan: cleanup multiple unregistrations

The temporary copy of the VLAN group is not neccessary since the lower device
is already in the process of being unregistered, if it was neccessary the
memset of the global group would introduce a race condition.

With this removed, the changes to the original code are only a few lines, so
remove the new function and move the code back into vlan_device_event().

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>