Why don't I have the option to delete screen schemes?

[pfoley]

I deleted a Service Desk project, but I keep finding artifacts of it in my Jira instance. I'm deleting them as I find them, but on the Issues -> Screen Schemes, I only seen buttons to Configure, Edit, or Copy the now-unnecessary schemes, not to Delete them. They aren't associated with any workflows or transitions as far as I can tell. Why am I not seeing a Delete button here?

[Raynard Rhodes]

I get the option to delete a newly created screen scheme. It's not associated to an issue type scheme.