[ML] Remove deprecated sort from reindex operation (elastic#117606)

Sort in reindex is deprecated. This PR removes its use from within the reindexing step of dataframe analytics.

Testing indicates that having the destination index sorted is a "nice to have" and not necessary for the DFA functionality to succeed.

docs/changelog/117606.yaml

@@ -0,0 +1,5 @@
+pr: 117606
+summary: Remove deprecated sort from reindex operation within dataframe analytics procedure
+area: Machine Learning
+type: enhancement
+issues: []

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/dataframe/steps/ReindexingStep.java

@@ -27,13 +27,11 @@
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.IndexNotFoundException;
-import org.elasticsearch.index.mapper.SeqNoFieldMapper;
 import org.elasticsearch.index.reindex.BulkByScrollResponse;
 import org.elasticsearch.index.reindex.BulkByScrollTask;
 import org.elasticsearch.index.reindex.ReindexAction;
 import org.elasticsearch.index.reindex.ReindexRequest;
 import org.elasticsearch.script.Script;
-import org.elasticsearch.search.sort.SortOrder;
 import org.elasticsearch.tasks.Task;
 import org.elasticsearch.tasks.TaskCancelledException;
 import org.elasticsearch.tasks.TaskId;
@@ -147,7 +145,6 @@ protected void doExecute(ActionListener<StepResponse> listener) {
             reindexRequest.setSourceQuery(config.getSource().getParsedQuery());
             reindexRequest.getSearchRequest().allowPartialSearchResults(false);
             reindexRequest.getSearchRequest().source().fetchSource(config.getSource().getSourceFiltering());
-            reindexRequest.getSearchRequest().source().sort(SeqNoFieldMapper.NAME, SortOrder.ASC);
             reindexRequest.setDestIndex(config.getDest().getIndex());
 
             // We explicitly set slices to 1 as we cannot parallelize in order to have the incremental id