Remove if prepared_at check in new worker

What does this MR do and why?

Currently this line is redundant because we don't have any where which is rerunning this new merge request worker. In the future we will rerun it if the prepared is null, but for now that functionality does not exist.

We want to backfill the prepared_at in !111865 (merged), but this may result in us setting a prepared_at when its not actually yet prepared, and with this method we would not run NewMergeRequestWorker in that case, however we still want to run it.

So we temporarily remove this check until after the backfill is run on .com.