remove assertions on the return value of delete_all_flows

This function can no longer fail.
diff --git a/src/python/oftest/testutils.py b/src/python/oftest/testutils.py
index 0fc6ce0..cb6bf3a 100644
--- a/src/python/oftest/testutils.py
+++ b/src/python/oftest/testutils.py
@@ -44,7 +44,7 @@
     msg.command = ofp.OFPFC_DELETE
     msg.buffer_id = 0xffffffff
     ctrl.message_send(msg)
-    return 0
+    return 0 # for backwards compatibility
 
 def required_wildcards(parent):
     w = test_param_get('required_wildcards', default='default')
@@ -571,8 +571,7 @@
 
     if clear_table: 
         logging.debug("Clear flow table")
-        rc = delete_all_flows(parent.controller)
-        parent.assertEqual(rc, 0, "Failed to delete all flows")
+        delete_all_flows(parent.controller)
 
     logging.debug("Insert flow")
     parent.controller.message_send(request)