use subnet_service_types extension from neutron-lib
use subnet_service_types extension from neutron-lib

This patch switches over to use the subnet_service_types extension
from neutron-lib's API definitions by removing the extension and
test module and using the lib API definition instead.

NeutronLibImpact

Change-Id: Ibba7af42aef7403472a11abe7b0e2e9239c4958e

mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron