Revert "Re-enable some performance updates to ES|QL spec tests after resilience improvement to EsqlSpecTestCase (#136781)" (#137589)

This reverts commit 4d3e27f.

Yesterday we merged two performance updates to ES|QL Spec tests that had
been reverted two months back after noticing some increased test
flakiness. However, we immediately saw increased failures due to one of
them, so we reverted it. Having done that we continue to see other
failures, that may or may not be related to the remaining performance
update. This PR reverts that too.

Author: craigtaverner

x-pack/plugin/esql/qa/server/multi-clusters/src/javaRestTest/java/org/elasticsearch/xpack/esql/ccq/MultiClusterSpecIT.java

@@ -15,7 +15,6 @@
 import org.elasticsearch.Version;
 import org.elasticsearch.client.Request;
 import org.elasticsearch.client.Response;
-import org.elasticsearch.client.ResponseListener;
 import org.elasticsearch.client.RestClient;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.core.IOUtils;
@@ -39,8 +38,6 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Set;
-import java.util.concurrent.CompletableFuture;
-import java.util.concurrent.ExecutionException;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 
@@ -254,7 +251,10 @@ static RestClient twoClients(RestClient localClient, RestClient remoteClient) th
                     return bulkClient.performRequest(request);
                 } else {
                     Request[] clones = cloneRequests(request, 2);
-                    return runInParallel(localClient, remoteClient, clones);
+                    Response resp1 = remoteClient.performRequest(clones[0]);
+                    Response resp2 = localClient.performRequest(clones[1]);
+                    assertEquals(resp1.getStatusLine().getStatusCode(), resp2.getStatusLine().getStatusCode());
+                    return resp2;
                 }
         });
         doAnswer(invocation -> {
@@ -289,44 +289,6 @@ static Request[] cloneRequests(Request orig, int numClones) throws IOException {
         return clones;
     }
 
-    /**
-     * Run {@link #cloneRequests cloned} requests in parallel.
-     */
-    static Response runInParallel(RestClient localClient, RestClient remoteClient, Request[] clones) throws Throwable {
-        CompletableFuture<Response> remoteResponse = new CompletableFuture<>();
-        CompletableFuture<Response> localResponse = new CompletableFuture<>();
-        remoteClient.performRequestAsync(clones[0], new ResponseListener() {
-            @Override
-            public void onSuccess(Response response) {
-                remoteResponse.complete(response);
-            }
-
-            @Override
-            public void onFailure(Exception exception) {
-                remoteResponse.completeExceptionally(exception);
-            }
-        });
-        localClient.performRequestAsync(clones[1], new ResponseListener() {
-            @Override
-            public void onSuccess(Response response) {
-                localResponse.complete(response);
-            }
-
-            @Override
-            public void onFailure(Exception exception) {
-                localResponse.completeExceptionally(exception);
-            }
-        });
-        try {
-            Response remote = remoteResponse.get();
-            Response local = localResponse.get();
-            assertEquals(remote.getStatusLine().getStatusCode(), local.getStatusLine().getStatusCode());
-            return local;
-        } catch (ExecutionException e) {
-            throw e.getCause();
-        }
-    }
-
     /**
      * Convert FROM employees ... => FROM *:employees,employees
      */