[TEST][Monitoring] Remove Unneeded Assert (elastic/x-pack-elasticsearch#4284)

This removes the `assert` that has been causing some very unexpected test
failures due to unexpected threading issues.

Some of the code changed and became async, so it is no longer guaranteed
that the same thread calls `doFlush` and `doClose`. We could similarly
make the field `volatile`, but since this `assert` is not really helping
anything it's easier to remove it.

Original commit: elastic/x-pack-elasticsearch@ba39de605f

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

@@ -123,8 +123,6 @@ class HttpExportBulk extends ExportBulk {
 
     @Override
     protected void doClose(ActionListener<Void> listener) {
-        // nothing serious to do at this stage
-        assert payload == null;
         listener.onResponse(null);
     }