Remove func not in use in test_service_providers

This patch removes an unused function _add_cleanup() from
this plugin.

Change-Id: I3647b639618fcebc99a9e7275d80b87b034e2960
diff --git a/keystone_tempest_plugin/tests/api/identity/v3/test_service_providers.py b/keystone_tempest_plugin/tests/api/identity/v3/test_service_providers.py
index 47a1c09..26110f8 100644
--- a/keystone_tempest_plugin/tests/api/identity/v3/test_service_providers.py
+++ b/keystone_tempest_plugin/tests/api/identity/v3/test_service_providers.py
@@ -48,10 +48,6 @@
                 self.assertEqual(
                     sp_ref['relay_state_prefix'], sp['relay_state_prefix'])
 
-    def _add_cleanup(self, sp_id):
-        self.addCleanup(
-            self.sps_client.delete_service_provider, sp_id)
-
     def _create_sp(self, sp_id, sp_ref):
         sp = self.sps_client.create_service_provider(
             sp_id, **sp_ref)['service_provider']