other のトリガーに触れるのをやめたときに OnTriggerExit は呼び出されます。
This message is sent to the trigger and the Collider that touches the trigger. Notes: Trigger events are only sent if one of the Colliders also has a Rigidbody attached. Trigger events will be sent to disabled MonoBehaviours, to allow enabling Behaviours in response to collisions. OnTriggerExit occurs on the FixedUpdate after the Colliders have stopped touching. The Colliders involved are not guaranteed to be at the point of initial separation.