bugfix: remove NSCoding archive after new game is started
bugfix: remove NSCoding archive after new game is started

this fixes the following scenario:
1) start new game + play a few moves
2) start another new game
3) suspend + kill the app
4) relaunch the app

- expected: app presents a new game created in 2)
- actual: app restores game state from 1) because NSCoding archive is still present

herzbube/littlego,herzbube/littlego,herzbube/littlego,herzbube/littlego,puremourning/littlego,puremourning/littlego,puremourning/littlego,herzbube/littlego,puremourning/littlego