Removes 'exchange' and 'backstab' objectives

Fixes https://github.com/NTStation/NTstation13/issues/704

I know testing random objectives is hard but shit this basically breaks traitor rounds and has been for a while now. Traitors without objectives, traitors not appearing at the endgame screen, etc.

If it actually gets fixed ikarrus can undo this but right now, since traitor is one of our most played game modes, it's going to have to be removed.