Remove unused _add_router_interface_with_subnet_id helper

_add_router_interface_with_subnet_id is not unused in
RoutersAdminTest, so this is to remove it.

Change-Id: I6945ed815580982ea68a76ad677a282c096295b2
diff --git a/tempest/api/network/admin/test_routers.py b/tempest/api/network/admin/test_routers.py
index c97efe9..f745f9c 100644
--- a/tempest/api/network/admin/test_routers.py
+++ b/tempest/api/network/admin/test_routers.py
@@ -41,19 +41,6 @@
         self.addCleanup(self._cleanup_router, router)
         return router
 
-    def _add_router_interface_with_subnet_id(self, router_id, subnet_id):
-        interface = self.routers_client.add_router_interface(
-            router_id, subnet_id=subnet_id)
-        self.addCleanup(self._remove_router_interface_with_subnet_id,
-                        router_id, subnet_id)
-        self.assertEqual(subnet_id, interface['subnet_id'])
-        return interface
-
-    def _remove_router_interface_with_subnet_id(self, router_id, subnet_id):
-        body = self.routers_client.remove_router_interface(router_id,
-                                                           subnet_id=subnet_id)
-        self.assertEqual(subnet_id, body['subnet_id'])
-
     @classmethod
     def skip_checks(cls):
         super(RoutersAdminTest, cls).skip_checks()