libvirt: remove VIF driver classes deprecated in Icehouse

In Icehouse the old VIF driver classes were set as deprecated
to be removed in Juno. Now they can be finally deleted.

Related-bug: #1302796
Change-Id: I7be735062e6cc6aa54dec1bf43d3cce1ac544988
Merge "libvirt: remove VIF driver classes deprecated in Icehouse"