excluded vxlan interface from physical network mappings

This PR removes the vxlan physical interface mappings in the ml2_conf.ini, which is not needed (Please double check me on this)
It could also cause problems with starting neutron when running RPC on a consolidated architecture i.e. running all traffic over br-mgmt, in which case the mappings would be vxlan:br-mgmt,vlan:br-mgmt. The neutron agent would then throw an error like: 

ERROR neutron.plugins.linuxbridge.agent.linuxbridge_neutron_agent [-] Parsing physical_interface_mappings failed: Value br-mgmt in mapping: 'vxlan:br-mgmt' not unique. Agent terminated!

-Alex