api-ref: remove 'external_network_bridge' from agent

After the neutron server removes this option [1], it will be removed
from the response of agent endpoint.

[1] https://review.openstack.org/#/c/567369/

Depends-On: I07474713206c218710544ad98c08caaa37dbf53a
Change-Id: I2ca0ef560484dff384b3a3b477456c9fe4352b8b

api-ref/source/v2/parameters.yaml

@@ -5386,12 +5386,6 @@ router-external_gateway_ports:
   in: body
   required: true
   type: string
-router-external_network_bridge:
-  description: |
-   Router external network bridge
-  in: body
-  required: true
-  type: string
 router-flavor_id:
   description: |
     The ID of the flavor associated with the router.

api-ref/source/v2/samples/agents/agents-list-response.json

@@ -100,7 +100,6 @@
                 "floating_ips": 0,
                 "interface_driver": "openvswitch",
                 "log_agent_heartbeats": false,
-                "external_network_bridge": "",
                 "ex_gw_ports": 1
             }
         },

api-ref/source/v2/samples/agents/router-l3-agent-list-response.json

@@ -24,9 +24,8 @@
                 "floating_ips": 0,
                 "interface_driver": "openvswitch",
                 "log_agent_heartbeats": false,
-                "external_network_bridge": "",
                 "ex_gw_ports": 1
             }
         }
     ]
-}
+}