Abstract toString into AbstractNode, deprecate the implementation

Log that the default implementation will be removed

pmd-test/src/main/java/net/sourceforge/pmd/test/lang/ast/DummyNode.java

@@ -11,6 +11,7 @@ public class DummyNode extends AbstractNode {
         super(id);
     }
 
+    @Deprecated
     @Override
     public String toString() {
         return "dummyNode";