Remove allow_overlapping_ips configuration option

This option was removed from Neutron in the Zed release [1]. This can be
backported to Yoga where the default value was changed to True [2].

[1] https://review.opendev.org/c/openstack/neutron/+/837286
[2] https://review.opendev.org/c/openstack/neutron/+/807848

Change-Id: Ibcd81a3a5f4b8de60459b3a4cfc30a50a06a436f