Great question. So yeah not all sites list the coordinator. In that case, I wouldn't feel the need to necessarily withdrawal from those programs. Let them take the lead, if they email you to invite you for an interview kindly let them know that you have already accepted another offer. They won't be upset and at that point it's still early enough for them that they didn't really lose out on anything (aside from a little time reading your application).
This is what I did when I couldn't find a coordinator or I didn't know someone at the program.