Neutron-legacy: Remove LINUXNET_VIF_DRIVER option

A value is never assigned, and it ends up in the nova.conf file as:

linuxnet_interface_driver =

So, let's delete it.

Change-Id: Ibc270ce6ee622eee871df1f8c11f21e8be8280ee
diff --git a/lib/neutron-legacy b/lib/neutron-legacy
index 4e51425..0968699 100644
--- a/lib/neutron-legacy
+++ b/lib/neutron-legacy
@@ -488,7 +488,6 @@
     # optionally set options in nova_conf
     neutron_plugin_create_nova_conf
 
-    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"
     fi