bridge: vlan: drop unnecessary flush code

As Ido Schimmel pointed out the vlan_vid_del() code in nbp_vlan_flush is
unnecessary (and is actually a remnant of the old vlan code) so we can
remove it.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>