This documentation about Management Project does not clearly provide details or information on how to delete a management project via API. Instead, it only states that management projects cannot be moved or deleted.

Google creates a Google-managed project in the folder. You cannot move or delete a management project.

Application Management for App Hub is still in Preview mode (Pre-GA), meaning that:

At Preview, products or features are ready for testing by customers. Preview offerings are often publicly announced, but are not necessarily feature-complete, and no SLAs or technical support commitments are provided for these. Unless stated otherwise by Google, Preview offerings are intended for use in test environments only. The average Preview stage lasts about six months.