With my extremely limited expertise, I would have to say that you cannot remove the letters from the schools that have received them already. Removing them won't change anything anyway. If you were placed on hold because you're missing the required letter (because that one doesn't qualify) you need to send another one. If the letter does not qualify the school will send an email letting you know. If you don't want to wait you can always contact them and ask them if you're missing something and they will tell you. There are plenty of schools that place on you hold simply because they are not offering you an interview during the first round, not because you don't qualify due to missing items.
