Tidy up ClusterStatsNodeRequest (#116036)

There was an `@UpdateForV9` on this class saying that it could be
replaced with `TransportRequest.Empty`. But that was removed by
#109790, and all its
non-test usages replaced by trivial implementations of
`TransportRequest`. This change therefore just simplifies the class a
bit and removes that annotation.

Author: PeteGillinElastic

server/src/main/java/org/elasticsearch/action/admin/cluster/stats/TransportClusterStatsAction.java

@@ -35,11 +35,9 @@
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.io.stream.StreamInput;
-import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.CancellableSingleObjectCache;
 import org.elasticsearch.common.util.concurrent.ThreadContext;
-import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.index.IndexService;
 import org.elasticsearch.index.engine.CommitStats;
 import org.elasticsearch.index.seqno.RetentionLeaseStats;
@@ -307,7 +305,6 @@ protected ClusterStatsNodeResponse nodeOperation(ClusterStatsNodeRequest nodeReq
         );
     }
 
-    @UpdateForV9(owner = UpdateForV9.Owner.DATA_MANAGEMENT) // this can be replaced with TransportRequest.Empty in v9
     public static class ClusterStatsNodeRequest extends TransportRequest {
 
         ClusterStatsNodeRequest() {}
@@ -320,11 +317,6 @@ public ClusterStatsNodeRequest(StreamInput in) throws IOException {
         public Task createTask(long id, String type, String action, TaskId parentTaskId, Map<String, String> headers) {
             return new CancellableTask(id, type, action, "", parentTaskId, headers);
         }
-
-        @Override
-        public void writeTo(StreamOutput out) throws IOException {
-            super.writeTo(out);
-        }
     }
 
     private static class MetadataStatsCache<T> extends CancellableSingleObjectCache<Metadata, Long, T> {