Remove unused _delete_router()

Since I95dafb51236b16474ec2d349334bb4339b0120ba the method has not
been used at all. Let's remove the method.

Change-Id: I945f5a1060b94a1d4f82fca3db629866076efa53
diff --git a/tempest/api/network/base_routers.py b/tempest/api/network/base_routers.py
index f6fd871..6d4e756 100644
--- a/tempest/api/network/base_routers.py
+++ b/tempest/api/network/base_routers.py
@@ -33,15 +33,6 @@
         self.addCleanup(self._cleanup_router, router)
         return router
 
-    def _delete_router(self, router_id, routers_client=None):
-        client = routers_client or self.routers_client
-        client.delete_router(router_id)
-        # Asserting that the router is not found in the list
-        # after deletion
-        list_body = client.list_routers()
-        routers_list = [router['id'] for router in list_body['routers']]
-        self.assertNotIn(router_id, routers_list)
-
     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)