Nick, sorry to tell you but in my experience (it happened to me) it is there to stay.
If there is a way to remove it, I do not know what it is. For future reference, this is why B&S mentions using a sheet of mylar in between the negative and the coated paper. Seem to remember Richard Sullivian saying he never had an issue with it, but it was a possilbility. I did, and ended up using the mylar to keep from having it happen again. Of course the trick is to get the coated paper dry enough that this will not happen, but not too dry as the process uses humidity in the coated paper.
Sorry...