Revert "[8.15] [ML] Downloaded and write model parts using multiple streams (#112869)" (#113017)

This reverts commit f48a1c6.

Author: davidkyle

docs/changelog/111684.yaml

@@ -9,23 +9,9 @@
 import java.util.Locale;
 
 public enum Level {
-    INFO {
-        public org.apache.logging.log4j.Level log4jLevel() {
-            return org.apache.logging.log4j.Level.INFO;
-        }
-    },
-    WARNING {
-        public org.apache.logging.log4j.Level log4jLevel() {
-            return org.apache.logging.log4j.Level.WARN;
-        }
-    },
-    ERROR {
-        public org.apache.logging.log4j.Level log4jLevel() {
-            return org.apache.logging.log4j.Level.ERROR;
-        }
-    };
-
-    public abstract org.apache.logging.log4j.Level log4jLevel();
+    INFO,
+    WARNING,
+    ERROR;
 
     /**
      * Case-insensitive from string method.

docs/changelog/112869.yaml

@@ -15,17 +15,12 @@
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
 import org.elasticsearch.common.settings.Setting;
-import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.plugins.ActionPlugin;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.tasks.Task;
-import org.elasticsearch.threadpool.ExecutorBuilder;
-import org.elasticsearch.threadpool.FixedExecutorBuilder;
 import org.elasticsearch.xpack.core.ml.packageloader.action.GetTrainedModelPackageConfigAction;
 import org.elasticsearch.xpack.core.ml.packageloader.action.LoadTrainedModelPackageAction;
 import org.elasticsearch.xpack.ml.packageloader.action.ModelDownloadTask;
-import org.elasticsearch.xpack.ml.packageloader.action.ModelImporter;
 import org.elasticsearch.xpack.ml.packageloader.action.TransportGetTrainedModelPackageConfigAction;
 import org.elasticsearch.xpack.ml.packageloader.action.TransportLoadTrainedModelPackage;
 
@@ -49,15 +44,16 @@ public class MachineLearningPackageLoader extends Plugin implements ActionPlugin
         Setting.Property.Dynamic
     );
 
+    // re-using thread pool setup by the ml plugin
+    public static final String UTILITY_THREAD_POOL_NAME = "ml_utility";
+
     // This link will be invalid for serverless, but serverless will never be
     // air-gapped, so this message should never be needed.
     private static final String MODEL_REPOSITORY_DOCUMENTATION_LINK = format(
         "https://www.elastic.co/guide/en/machine-learning/%s/ml-nlp-elser.html#air-gapped-install",
         Build.current().version().replaceFirst("^(\\d+\\.\\d+).*", "$1")
     );
 
-    public static final String MODEL_DOWNLOAD_THREADPOOL_NAME = "model_download";
-
     public MachineLearningPackageLoader() {}
 
     @Override
@@ -85,24 +81,6 @@ public List<NamedWriteableRegistry.Entry> getNamedWriteables() {
         );
     }
 
-    @Override
-    public List<ExecutorBuilder<?>> getExecutorBuilders(Settings settings) {
-        return List.of(modelDownloadExecutor(settings));
-    }
-
-    public static FixedExecutorBuilder modelDownloadExecutor(Settings settings) {
-        // Threadpool with a fixed number of threads for
-        // downloading the model definition files
-        return new FixedExecutorBuilder(
-            settings,
-            MODEL_DOWNLOAD_THREADPOOL_NAME,
-            ModelImporter.NUMBER_OF_STREAMS,
-            -1, // unbounded queue size
-            "xpack.ml.model_download_thread_pool",
-            EsExecutors.TaskTrackingConfig.DO_NOT_TRACK
-        );
-    }
-
     @Override
     public List<BootstrapCheck> getBootstrapChecks() {
         return List.of(new BootstrapCheck() {