remove assertions on the return value of delete_all_flows

This function can no longer fail.
diff --git a/tests/flow_expire.py b/tests/flow_expire.py
index 705cab0..ece6dcd 100644
--- a/tests/flow_expire.py
+++ b/tests/flow_expire.py
@@ -37,8 +37,7 @@
         of_ports.sort()
         self.assertTrue(len(of_ports) > 1, "Not enough ports for test")
 
-        rc = delete_all_flows(self.controller)
-        self.assertEqual(rc, 0, "Failed to delete all flows")
+        delete_all_flows(self.controller)
 
         pkt = simple_tcp_packet()
         match = packet_to_flow_match(self, pkt)