devref: Remove stale description about network_ip_availability
devref: Remove stale description about network_ip_availability

You no longer need to enable the plugin explicitly
after the recent change. [1]

[1] I8c1568d097525c43576e516bd86d820c6b0a78fb

Change-Id: I7c546e199e1e71708285e66345f650dbaff681b2

huntxu/neutron,eayunstack/neutron,openstack/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,mahak/neutron,sebrandon1/neutron,huntxu/neutron,mahak/neutron,igor-toga/local-snat,cloudbase/neutron,sebrandon1/neutron,eayunstack/neutron,igor-toga/local-snat,cloudbase/neutron,openstack/neutron,noironetworks/neutron