Yes it is possible to get rid of all the buildings in the mission. But only the buildings that were added there by mission editor. And you'd be using a maphack. Im sure you can find out how to do that, I've seen a couple strings on this forum. Like Mullah Omar said, if you can get a world editor (
www.ofp.info - utilities) you can delete all the trees and buildings and so on. But just remember when you connect to a server it will say that you have a modified world file. But a lot of people have this error so I'm sure it will go unnoticed.