remove assertions on the return value of delete_all_flows

This function can no longer fail.
diff --git a/tests/flow_stats.py b/tests/flow_stats.py
index e8322fd..a96bb45 100644
--- a/tests/flow_stats.py
+++ b/tests/flow_stats.py
@@ -119,8 +119,7 @@
         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)
 
         # build packet
         pkt = simple_tcp_packet()
@@ -259,8 +258,7 @@
         egress_port1 = of_ports[1];
         egress_port2 = of_ports[2];
 
-        rc = delete_all_flows(self.controller)
-        self.assertEqual(rc, 0, "Failed to delete all flows")
+        delete_all_flows(self.controller)
 
         pkt1 = simple_tcp_packet()
         flow_mod_msg1 = self.buildFlowModMsg(pkt1, ingress_port, egress_port1)
@@ -371,8 +369,7 @@
         egress_port1 = of_ports[1];
         egress_port2 = of_ports[2];
 
-        rc = delete_all_flows(self.controller)
-        self.assertEqual(rc, 0, "Failed to delete all flows")
+        delete_all_flows(self.controller)
 
         pkt1 = simple_tcp_packet()
         flow_mod_msg1 = self.buildFlowModMsg(pkt1, ingress_port, egress_port1)
@@ -415,8 +412,7 @@
     the query doesn't match any flows.
     """
     def runTest(self):
-        rc = delete_all_flows(self.controller)
-        self.assertEqual(rc, 0, "Failed to delete all flows")
+        delete_all_flows(self.controller)
         match = ofp.ofp_match()
         match.wildcards = 0
         stat_req = message.flow_stats_request()
@@ -436,8 +432,7 @@
     the query doesn't match any flows.
     """
     def runTest(self):
-        rc = delete_all_flows(self.controller)
-        self.assertEqual(rc, 0, "Failed to delete all flows")
+        delete_all_flows(self.controller)
         match = ofp.ofp_match()
         match.wildcards = 0
         stat_req = message.aggregate_stats_request()
@@ -473,8 +468,7 @@
         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)
 
         # build packet
         pkt = simple_tcp_packet()
@@ -515,8 +509,7 @@
 
         # delete flow
         logging.info("Deleting flow")
-        rc = delete_all_flows(self.controller)
-        self.assertEqual(rc, 0, "Failed to delete all flows")
+        delete_all_flows(self.controller)
 
         # wait for flow_removed message
         flow_removed, _ = self.controller.poll(