The current way for immediate agent removal is not supported via SCOM SDK, but involves some SQL tweaking, in order to cleanup the database in advance from remnants of the object. The reason is that all discovery workflows should pass (on their respective schedule) before all references to the objects are removed. More details here: https://kevinholman.com/2018/05/03/deleting-and-purging-data-from-the-scom-database/