Remove PipelineExecutionService#executeIndexRequest (#29537)

With the move long ago to execute all single-document indexing requests
as bulk indexing request, the method
PipelineExecutionService#executeIndexRequest is unused and will never be
used in production code. This commit removes this method and cuts over
all tests to use PipelineExecutionService#executeBulkRequest.

Author: jasontedor

server/src/main/java/org/elasticsearch/ingest/PipelineExecutionService.java

@@ -53,23 +53,6 @@ public PipelineExecutionService(PipelineStore store, ThreadPool threadPool) {
         this.threadPool = threadPool;
     }
 
-    public void executeIndexRequest(IndexRequest request, Consumer<Exception> failureHandler, Consumer<Boolean> completionHandler) {
-        Pipeline pipeline = getPipeline(request.getPipeline());
-        threadPool.executor(ThreadPool.Names.INDEX).execute(new AbstractRunnable() {
-
-            @Override
-            public void onFailure(Exception e) {
-                failureHandler.accept(e);
-            }
-
-            @Override
-            protected void doRun() throws Exception {
-                innerExecute(request, pipeline);
-                completionHandler.accept(true);
-            }
-        });
-    }
-
     public void executeBulkRequest(Iterable<DocWriteRequest> actionRequests,
                                    BiConsumer<IndexRequest, Exception> itemFailureHandler,
                                    Consumer<Exception> completionHandler) {