Revert YARN-4126. RM should not issue delegation tokens in unsecure mode.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/ClientRMService.java

@@ -1238,7 +1238,7 @@ public class ClientRMService extends AbstractService implements
           .contains(UserGroupInformation.getCurrentUser()
                   .getRealAuthenticationMethod());
     } else {
-      return false;
+      return true;
     }
   }