Fixed issue with polyominoes not moving down after row clearing.
Game can only end as a result of the current polyomino being placed.

So we don't need to pass in a polyomino to the is_game_over function.
I don't remember why this is an issue.

The current_polyomino isn't on the board, but that's fine as it's tested for collisions using the board, then gets added when it gets placed.