Since the introduction of Windows Autopilot in Intune, Intune administrators know it is not easy step to remove a Windows Autopilot registration.
You had first to remove the device from Intune and then delete the registration.
Well, good news, this process has been easier by simply deleting the Windows Autopilot registration from the Windows Autopilot devices blade.
When deleting the registration, if the device is no longer active in Intune, the corresponding object is also deleted BUT if the device is still active, only the registration is removed, the device is kept under Intune management.