I know this has been resolved before I got to it, but I wanted to chime in in case someone was reading it who wanted to withdraw from a more typical search (i.e. no extra essay) following a phone or even campus interview.
In my mind, I really don't see the point of waiting for them to invite you to the next step before you tell them something. Search committees usually have to weed from 10 to 3-4 candidates from phone to in person interviews. They don't necessarily at that time rank every single of the 10 candidates; they might just select the ones invited and keep the others on ice but without a specific order in mind. So if you drop out AFTER they have decided to invite you, you could be forcing the committee into another arduous meeting to select your alternate. And even putting that meeting aside (lets say they do rank a few alternates in the first meeting), it is so much easier for them to go from 9 to 3-4 than it is to go from 10 to 3-4. It really is. Why make them think about you if you are done with them? Just tell them you want to drop out if you want to drop out! I honestly can't think of a single advantage to you or them for you to drop out earlier than that.
And if you're dropping out after a campus interview…even more so…do it as soon as you know, irrespective of whether you've been given the offer.