Unable to Delete Issue Type Screen Scheme due to Missing Delete Button
In the Issue Type Screen Schemes page, there is an scheme that is missing the "delete" button and is not associated with any projects.
A possibility is that the Default Issue Type Screen Scheme had been renamed or replaced. This original default scheme is not deleteable, and will remain that way even when renaming it. To confirm if this is the case hover the mouse over the scheme name in question. If the URL ends with "id=1", then that scheme is the Default Issue Type Screen Scheme.
As the default scheme is undeleteable, it is recommended to use it if only using one scheme. All other schemes can be deleted when disassociated them from all projects.