Fix syntax for a del() call

Change-Id: I07f0020d9d66a2fe763fd17215c2f08d86e2ed65
diff --git a/util.py b/util.py
index 1f25f6b..1bb0534 100644
--- a/util.py
+++ b/util.py
@@ -28,7 +28,7 @@
             for name in s.switch_get_port_names():
                 ret[switch_name] += '%s ' % name
             s.switch_disconnect()
-            del(s)
+            del s
         return ret
 
     # Simple helper wrapper for all the read-only database queries