Use execute() instead of submit() in MlMemoryTracker

We don't need a Future to wait for completion

Author: David Roberts

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/process/MlMemoryTracker.java

@@ -225,7 +225,7 @@ private void iterateMlJobTasks(Iterator<PersistentTasksCustomMetaData.Persistent
                     // can occur if the searches happen to be on the local node, as the huge
                     // chain of listeners are all called in the same thread if only one node
                     // is involved
-                    mem -> threadPool.executor(executorName()).submit(() -> iterateMlJobTasks(iterator, refreshComplete)),
+                    mem -> threadPool.executor(executorName()).execute(() -> iterateMlJobTasks(iterator, refreshComplete)),
                     refreshComplete::onFailure));
         } else {
             refreshComplete.onResponse(null);