DVR: move delete_floatingip_agent_gateway_port
DVR: move delete_floatingip_agent_gateway_port

This relocates some code to a different part of
the module in preparation for a class split. There is
no logic change.

Functions moved:
* delete_floatingip_agent_gateway_port

Related: blueprint multi-l3-backends
Change-Id: I889703f42419afdfc832c5acb76edb327d3e099f

openstack/neutron,openstack/neutron,noironetworks/neutron,huntxu/neutron,huntxu/neutron,openstack/neutron,eayunstack/neutron,mahak/neutron,mahak/neutron,mahak/neutron,eayunstack/neutron,noironetworks/neutron