remove unsupported libvirt_vif_driver nova.conf setting

This was deprecated in Havana, and unsupported in Icehouse since:
https://review.openstack.org/#/c/49660/

This causes a nova compute service startup failure
when setting up OVS with neutron (the default setup).

Change-Id: I7419bea9443dae353ac46191a6d66704b14e3971
Merge "remove unsupported libvirt_vif_driver nova.conf setting"