Remove Secret entity

It looks to me as if:
```dart
/// A secret value stored in Datastore, typically an access credential used by
/// the application.
@db.Kind(name: 'Secret', idType: db.IdType.String)
class Secret extends db.Model {
  @db.StringProperty(required: true)
  String? value;
}
```

isn't being used anymore. We probably have to first use a background job to remove all the entities.
