A scene saves a camera angle and layer state you captured earlier. See What is a Scene? for exactly what it stores.
Delete one scene
Hover over a scene and click the trash icon that appears, or right-click the scene and choose Delete.
Delete all scenes
Select the ... menu in Scenes, then select "Delete All."
🚨 Warning: Delete all, in the ••• menu, removes every scene in the panel at once. Confirm before choosing it, since deleted scenes cannot be recovered.