[ML] Remove deprecated sort from reindex operation

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

Although this doesn't appear to have broken fundamental functionality,
creating this PR as a draft initially to smoke out any unforeseen
consequences.

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

@@ -147,7 +147,6 @@ 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