With the current implementation, phydev cannot be removed:
$ ip link add dummy type dummy
$ ip link add xfrm1 type xfrm dev dummy if_id 1
$ ip l d dummy
kernel:[77938.465445] unregister_netdevice: waiting for dummy to become free. Usage count = 1
Manage it like in ip tunnels, ie just keep the ifindex. Not that the side
effect, is that the phydev is now optional.