Router appended twice to routers list when created with create_router_by_client

create_router_by_client creates a router by calling either create_router
or create_admin_router
In all these cases, the router is appened twice to the routers list,
i.e. the router is duplicated in that list

Change-Id: I1844abc9d5f14611141b19e9003077e2396fba2a
diff --git a/neutron_tempest_plugin/scenario/base.py b/neutron_tempest_plugin/scenario/base.py
index 752e536..127701c 100644
--- a/neutron_tempest_plugin/scenario/base.py
+++ b/neutron_tempest_plugin/scenario/base.py
@@ -205,7 +205,6 @@
         else:
             router = cls.create_admin_router(**kwargs)
         LOG.debug("Created router %s", router['name'])
-        cls.routers.append(router)
         return router
 
     @removals.remove(version='Stein',