Fixed bugs regarding despawning of blocks.

The bug was caused by the World::mNumberOfDestroyedEnemies variable being decremented twice (should be once) whenever a tank was despawned because I changed the if-statement condition in World::updateEnemyCounters() from Tank::IsMarkedForRemoval() to Tank::IsDestroyed(). I removed the bug by switching it back to Tank::IsMarkedForRemoval().