[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>

include/linux/if_vlan.h

@@ -16,11 +16,6 @@
 #ifdef __KERNEL__
 
 /* externally defined structs */
-struct vlan_group;
-struct net_device;
-struct packet_type;
-struct vlan_collection;
-struct vlan_dev_info;
 struct hlist_node;
 
 #include <linux/netdevice.h>