ONOS-2294: Redefine LinkResource as interface

LinkResource was defined as an abstract class, but it doesn't define any
methods. Maker interface fits this case.

Note: This change may break backward compatibility for those defining a
sub-class of LinkResource.

Change-Id: I53cb7e8eae7057d07c63960c488979a99cd54d52
diff --git a/core/api/src/main/java/org/onosproject/net/resource/link/LambdaResource.java b/core/api/src/main/java/org/onosproject/net/resource/link/LambdaResource.java
index 7ed8847..3733e46 100644
--- a/core/api/src/main/java/org/onosproject/net/resource/link/LambdaResource.java
+++ b/core/api/src/main/java/org/onosproject/net/resource/link/LambdaResource.java
@@ -24,7 +24,7 @@
 /**
  * Representation of lambda resource.
  */
-public final class LambdaResource extends LinkResource {
+public final class LambdaResource implements LinkResource {
 
     private final IndexedLambda lambda;