[OVN] Follow up of OVN module migration to DevStack

This patch is a follow-up of Ib4194329474e8d68a90886d2a04f027eecd741df.

This patch removes the configure_port_forwarding call from the
neutron-legacy module because port forwarding (just like other
extensions such as DNS, QOS, etc...) are already enabled in the
plugin.sh file in the neutron repository [0]. The
configure_port_forwarding method itself is also defined in the neutron
repository so calling it here may result in a failure in case the plugin
is not enabled.

We are also removing the "dns" extensions from the default
Q_ML2_PLUGIN_EXT_DRIVERS variable because this extension conflicts with
the default DNS extensions that is enabled by Neutron when
q-dns/neutron-dns service is enabled (also in [0]). The LP for this
conflict problem is: https://bugs.launchpad.net/neutron/+bug/1887163.

[0]
https://github.com/openstack/neutron/blob/945a244588b81064e4301b6f055a3c90f472bd7e/devstack/plugin.sh#L101-L103

Change-Id: Iafb9e45520798b2a612192cfc6cca28501465862
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>
diff --git a/lib/neutron_plugins/ovn_agent b/lib/neutron_plugins/ovn_agent
index e29b84f..f647f85 100644
--- a/lib/neutron_plugins/ovn_agent
+++ b/lib/neutron_plugins/ovn_agent
@@ -118,7 +118,7 @@
 Q_ML2_PLUGIN_TYPE_DRIVERS=${Q_ML2_PLUGIN_TYPE_DRIVERS:-local,flat,vlan,geneve}
 Q_ML2_TENANT_NETWORK_TYPE=${Q_ML2_TENANT_NETWORK_TYPE:-"geneve"}
 Q_ML2_PLUGIN_GENEVE_TYPE_OPTIONS=${Q_ML2_PLUGIN_GENEVE_TYPE_OPTIONS:-"vni_ranges=1:65536"}
-Q_ML2_PLUGIN_EXT_DRIVERS=${Q_ML2_PLUGIN_EXT_DRIVERS:-port_security,dns,qos}
+Q_ML2_PLUGIN_EXT_DRIVERS=${Q_ML2_PLUGIN_EXT_DRIVERS:-port_security,qos}
 # this one allows empty:
 ML2_L3_PLUGIN=${ML2_L3_PLUGIN-"ovn-router"}