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_plugins/brocade b/lib/neutron_plugins/brocade
index b8166d9..557b94d 100644
--- a/lib/neutron_plugins/brocade
+++ b/lib/neutron_plugins/brocade
@@ -12,7 +12,7 @@
 }
 
 function neutron_plugin_create_nova_conf {
-    NOVA_VIF_DRIVER=${NOVA_VIF_DRIVER:-"nova.virt.libvirt.vif.LibvirtGenericVIFDriver"}
+    :
 }
 
 function neutron_plugin_install_agent_packages {