Remove unused physical_network parameter in tripleo-image-elements

It was a leftover after `a7d02f115c71a00b6c4a1ffaf968481d6e0afec0` cleanup.

Change-Id: I76aaa2c1f4732d72d28943b44d0760089dda6dd0
Closes-Bug: 1345086

elements/neutron-openvswitch-agent/README.md

@@ -12,7 +12,6 @@ configured via Heat Metadata. For example:
         public_interface_raw_device: eth2
         public_interface_route: 12.34.56.78
         physical_bridge: br-ctlplane
-        physical_network: ctlplane
         network_vlan_ranges: ctlplane
         bridge_mappings: ctlplane:br-ctlplane
     bootstack:

elements/seed-stack-config/config.json

@@ -70,7 +70,6 @@
       "enable_tunneling": "False",
       "network_vlan_ranges": "ctlplane",
       "physical_bridge": "br-ctlplane",
-      "physical_network": "ctlplane",
       "public_interface": "eth1",
       "tenant_network_type": "vlan"
     },