[VLAN]: Remove unnecessary structure declarations

- struct packet_type is not used
- struct vlan_group is declared later in the file before the first use
- struct net_device is not needed since netdevice.h is included
- struct vlan_collection does not exist
- struct vlan_dev_info is declared later in the file before the first use

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