Revert "Bulletクラスの追加,Enemyとの当たり判定"

This reverts commit 6fbc260c3e819f6cc3d75e314dab3213af3e0c28.
Revert "Revert "Bulletクラスの追加,Enemyとの当たり判定""

This reverts commit bd0933c6461180418003c85ee6caf3af0a8b8845.