[neutron] OVN "remove_external_gateways" is making a duplicated call

The Neutron "remove_external_gateways" API call is making a duplicated call to the Neutron server.

This error affects only Neutron ML2/OVN plugin, since the implementation of [1].

Steps to reproduce:
* Create an external network. By default, using devstack, "public" network could be used
  $ openstack network create --external public
* Create a router:
  $ openstack router create router1
* Add a external GW network to the router:
  $ openstack set router --external-gateway public router1
* Remove the external gateway network:
  $ openstack unset router --external-gateway router1

Logs (in debug mode): https://paste.opendev.org/show/b7sErZU7ZqDDkmJ8IbK6/
* It first calls "remove_external_gateways" API.
* Then it calls the router update call with the same information.

[1]https://review.opendev.org/q/topic:%222023-aa-l3-gw-multihoming%22