Remove NOVA_VIF_DRIVER variable

Commit 7561c8ded211d53e8745d1420a73b82bd0fc35cf removed the
libvirt.vif_driver option from Nova in Juno so we should remove the
variable from devstack since it's useless / confusing.

Change-Id: I70a8cb4a3606eb5eabd3c0ef331945e72c80543a
diff --git a/lib/neutron-legacy b/lib/neutron-legacy
index da8c064..f381f6e 100644
--- a/lib/neutron-legacy
+++ b/lib/neutron-legacy
@@ -158,8 +158,6 @@
 Q_USE_DEBUG_COMMAND=${Q_USE_DEBUG_COMMAND:-False}
 # The name of the default q-l3 router
 Q_ROUTER_NAME=${Q_ROUTER_NAME:-router1}
-# nova vif driver that all plugins should use
-NOVA_VIF_DRIVER=${NOVA_VIF_DRIVER:-"nova.virt.libvirt.vif.LibvirtGenericVIFDriver"}
 Q_NOTIFY_NOVA_PORT_STATUS_CHANGES=${Q_NOTIFY_NOVA_PORT_STATUS_CHANGES:-True}
 Q_NOTIFY_NOVA_PORT_DATA_CHANGES=${Q_NOTIFY_NOVA_PORT_DATA_CHANGES:-True}
 VIF_PLUGGING_IS_FATAL=${VIF_PLUGGING_IS_FATAL:-True}
@@ -485,10 +483,9 @@
         iniset $NOVA_CONF DEFAULT security_group_api neutron
     fi
 
-    # set NOVA_VIF_DRIVER and optionally set options in nova_conf
+    # optionally set options in nova_conf
     neutron_plugin_create_nova_conf
 
-    iniset $NOVA_CONF libvirt vif_driver "$NOVA_VIF_DRIVER"
     iniset $NOVA_CONF DEFAULT linuxnet_interface_driver "$LINUXNET_VIF_DRIVER"
     if is_service_enabled q-meta; then
         iniset $NOVA_CONF neutron service_metadata_proxy "True"