Remove unnecessary resource_setup in DvrRoutersNegativeTest

In DvrRoutersNegativeTest's resource_setup, a router, a network,
and a subnet are created which is of no use. This is to remove
the unnecessary resource_setup to make the testcase faster and to
keep the code clean.

Change-Id: I146a7f4198b7fea53608e8e5cc51918fbeb8e3e5
diff --git a/tempest/api/network/test_routers_negative.py b/tempest/api/network/test_routers_negative.py
index 72face8..db165ab 100644
--- a/tempest/api/network/test_routers_negative.py
+++ b/tempest/api/network/test_routers_negative.py
@@ -128,13 +128,6 @@
             msg = "DVR extension not enabled."
             raise cls.skipException(msg)
 
-    @classmethod
-    def resource_setup(cls):
-        super(DvrRoutersNegativeTest, cls).resource_setup()
-        cls.router = cls.create_router()
-        cls.network = cls.create_network()
-        cls.subnet = cls.create_subnet(cls.network)
-
     @decorators.attr(type=['negative'])
     @decorators.idempotent_id('4990b055-8fc7-48ab-bba7-aa28beaad0b9')
     def test_router_create_tenant_distributed_returns_forbidden(self):