Remove neutron_metadata_socket volume from agents

Metadata Proxy UNIX domain socket is only used by neutron-l3-agent,
neutron-dhcp-agent and neutron-metadata-agent. Hence it must be removed
from neutron-bgp-agent, neutron-vpn-agent and neutron-lbaas-agent.

See comment here:
https://github.com/openstack/kolla-ansible/blob/master/ansible/roles/neutron/templates/neutron.conf.j2#L22

Change-Id: Ie493194a5dcd5979eb29382ecbd0c8db1b9e286d