veth: drop iface peer check during create_and_realize()

When fetching the parent device, if the system is slow, NetworkManager
can hit a race condition where the property is still NULL. In that case,
NetworkManager should create the veth link.

Checking that the peer device exists, it is type NM_DEVICE_TYPE_VETH and
it have a parent device is enough to know that we can skip the link
creation.

!1399

https://bugzilla.redhat.com/show_bug.cgi?id=2129829

Fixes: 4655b7c3 ('veth: fix veth activation on booting')