Remove _remove_router_interface_with_port_id()

_remove_router_interface_with_port_id() is just a wrapper method
which calls remove_router_interface() with an assertion.
The assertion seems unnecessary because it is called on the cleanup
and we don't do such check on the other places.
This patch just replaces the redunadnt method with direct call.

Change-Id: I82fd96be1f27dc9f9e58b5eeaa562ef3404f3382
diff --git a/tempest/api/network/base_routers.py b/tempest/api/network/base_routers.py
index 6d4e756..62062ba 100644
--- a/tempest/api/network/base_routers.py
+++ b/tempest/api/network/base_routers.py
@@ -45,8 +45,3 @@
         body = self.routers_client.remove_router_interface(router_id,
                                                            subnet_id=subnet_id)
         self.assertEqual(subnet_id, body['subnet_id'])
-
-    def _remove_router_interface_with_port_id(self, router_id, port_id):
-        body = self.routers_client.remove_router_interface(router_id,
-                                                           port_id=port_id)
-        self.assertEqual(port_id, body['port_id'])
diff --git a/tempest/api/network/test_routers.py b/tempest/api/network/test_routers.py
index 97ad4d4..ee0e395 100644
--- a/tempest/api/network/test_routers.py
+++ b/tempest/api/network/test_routers.py
@@ -153,8 +153,8 @@
         interface = self.routers_client.add_router_interface(
             router['id'],
             port_id=port_body['port']['id'])
-        self.addCleanup(self._remove_router_interface_with_port_id,
-                        router['id'], port_body['port']['id'])
+        self.addCleanup(self.routers_client.remove_router_interface,
+                        router['id'], port_id=port_body['port']['id'])
         self.assertIn('subnet_id', interface.keys())
         self.assertIn('port_id', interface.keys())
         # Verify router id is equal to device id in port details