Now there used to be a way,you could take it to some sort of thatcham approved centre where if the repair was to a certain standard then they could remove the marker but I'm not sure if that's still the case.tbh mate I don't know why anyone would want to bother as cat d could be something as stupid as the car being keyed all over and needing a full repaint.Also,if it's had heavyish damage it won't be too hard to tell and you'll only be trying to hide it from a seller who probably won't care anyway.