[ML] Adjust BWC for node in start/open responses following backport

Once elastic#55570 is merged the master branch BWC code is no longer needed.

Relates elastic#55473

Author: David Roberts

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/action/NodeAcknowledgedResponse.java

@@ -6,7 +6,6 @@
 
 package org.elasticsearch.xpack.core.ml.action;
 
-import org.elasticsearch.Version;
 import org.elasticsearch.action.support.master.AcknowledgedResponse;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
@@ -28,12 +27,7 @@ public NodeAcknowledgedResponse(boolean acknowledged, String node) {
 
     public NodeAcknowledgedResponse(StreamInput in) throws IOException {
         super(in);
-        // TODO change in backport
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
-            node = in.readString();
-        } else {
-            node = "";
-        }
+        node = in.readString();
     }
 
     public String getNode() {
@@ -43,10 +37,7 @@ public String getNode() {
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
-        // TODO change in backport
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
-            out.writeString(node);
-        }
+        out.writeString(node);
     }
 
     @Override

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/action/NodeAcknowledgedResponseTests.java

@@ -6,11 +6,10 @@
 
 package org.elasticsearch.xpack.core.ml.action;
 
-import org.elasticsearch.Version;
 import org.elasticsearch.common.io.stream.Writeable;
-import org.elasticsearch.xpack.core.ml.AbstractBWCWireSerializationTestCase;
+import org.elasticsearch.test.AbstractWireSerializingTestCase;
 
-public class NodeAcknowledgedResponseTests extends AbstractBWCWireSerializationTestCase<NodeAcknowledgedResponse> {
+public class NodeAcknowledgedResponseTests extends AbstractWireSerializingTestCase<NodeAcknowledgedResponse> {
 
     @Override
     protected NodeAcknowledgedResponse createTestInstance() {
@@ -30,14 +29,4 @@ protected NodeAcknowledgedResponse mutateInstance(NodeAcknowledgedResponse insta
             return new NodeAcknowledgedResponse(true, "");
         }
     }
-
-    @Override
-    protected NodeAcknowledgedResponse mutateInstanceForVersion(NodeAcknowledgedResponse instance, Version version) {
-        // TODO change in backport
-        if (version.onOrAfter(Version.V_8_0_0)) {
-            return instance;
-        } else {
-            return new NodeAcknowledgedResponse(true, "");
-        }
-    }
 }