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/LinkResource.java b/core/api/src/main/java/org/onosproject/net/resource/link/LinkResource.java
index 6cfa38d..ec06611 100644
--- a/core/api/src/main/java/org/onosproject/net/resource/link/LinkResource.java
+++ b/core/api/src/main/java/org/onosproject/net/resource/link/LinkResource.java
@@ -18,6 +18,5 @@
 /**
  * Abstraction of link resource.
  */
-public abstract class LinkResource {
-
+public interface LinkResource {
 }