Remove a link from seenLink when either end of the link goes down

Change-Id: Ie650f6fd34f10ed91112f974684e97b99c916534
diff --git a/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java b/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java
index 3bc56b6..6bacfce 100644
--- a/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java
+++ b/src/main/java/org/onosproject/segmentrouting/SegmentRoutingManager.java
@@ -1116,6 +1116,10 @@
                 .forEach(entry -> {
                     portNextObjStore.remove(entry.getKey());
                 });
+
+        seenLinks.keySet().removeIf(key -> key.src().deviceId().equals(device.id()) ||
+                key.dst().deviceId().equals(device.id()));
+
         groupHandlerMap.remove(device.id());
         defaultRoutingHandler.purgeEcmpGraph(device.id());
         // Note that a switch going down is associated with all of its links