You don't wanna do the PhD, you have your answer. You can drop the PhD, but it's dishonest and wrong to start the program knowing you don't want the PhD. Morally, Penn should not be an option for you. Somebody who wants to do the MSTP should be given that spot.
It is a huge headache for everybody to have an MSTP drop-out. You could maybe consider emailing the school and seeing if there's a way to only do the MD program from the beginning, but I doubt it. Probably wouldn't hurt to ask, though.