Removed the need to give NPCs fake objectives to properly end the game

This simplifies setting up single-player objectives a lot. The behavior outlined in https://github.com/OpenRA/OpenRA/pull/6377#issuecomment-53992565 was pretty wonky. I noticed while trying to fix https://github.com/OpenRA/OpenRA/issues/7068. When Tanya gets killed, it would automatically mark Einstein as killed and vice versa.
Don't know, but you are right. This should not be readonly.