network: stop recording saved physical net devices

network: stop recording saved physical net devices

liblxc will now correctly log any network device names and ifindeces in their
respective network namespaces. So there's no need to record physical network
devices any more. This spares us heap allocations and memory we need to have
lying around til the container is shutdown.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>