[Monitoring] HttpExportBulk shouldn't bother setting payload to null (elastic/x-pack-elasticsearch#4287)

This avoid setting the payload to `null` when sending it to the remote
monitoring cluster. The payload will be GCed when this overall object is
GCed, which should be very soon anyway.

Original commit: elastic/x-pack-elasticsearch@04f63c4150

plugin/monitoring/src/main/java/org/elasticsearch/xpack/monitoring/exporter/http/HttpExportBulk.java

@@ -96,9 +96,6 @@ class HttpExportBulk extends ExportBulk {
         } else if (payload.length != 0) {
             final HttpEntity body = new ByteArrayEntity(payload, ContentType.APPLICATION_JSON);
 
-            // free the memory
-            payload = null;
-
             client.performRequestAsync("POST", "/_bulk", params, body, new ResponseListener() {
                 @Override
                 public void onSuccess(Response response) {