[VOL-4495] Do not remove NNI flows, they are only wiped out on device disconnect

Change-Id: I904def137485f66a86fa9241a98e7ddeda10f481
diff --git a/impl/src/main/java/org/opencord/olt/impl/Olt.java b/impl/src/main/java/org/opencord/olt/impl/Olt.java
index 747a0b4..af3eed8 100644
--- a/impl/src/main/java/org/opencord/olt/impl/Olt.java
+++ b/impl/src/main/java/org/opencord/olt/impl/Olt.java
@@ -852,8 +852,12 @@
                 }
             } else {
                 if (oltDeviceService.isNniPort(device, port.number())) {
-                    // NOTE this may need to be handled on DEVICE_REMOVE as we don't disable the NNI
-                    oltFlowService.handleNniFlows(device, port, OltFlowService.FlowOperation.REMOVE);
+                    // NOTE the NNI is only disabled if the OLT shuts down (reboot or failure).
+                    // In that case the flows are purged anyway, so there's no need to deal with them,
+                    // it would actually be counter-productive as the openflow connection is severed and they won't
+                    // be correctly processed
+                    log.debug("NNI port went down, " +
+                            "ignoring event as flows will be removed in the generic device cleanup");
                 } else {
                     post(new AccessDeviceEvent(AccessDeviceEvent.Type.UNI_REMOVED, device.id(), port));
                     // NOTE we are assuming that if a subscriber has default eapol