I have some workflows I formerly assigned to certain issue types. After converting the respective issues to other issue types, I deleted the issue types as I don't need them anymore. Jira didn't complain.
However, Jira obviously didn't delete the assignment within the workflow scheme. There are now empty issue types that are assigned to certain workflows (the ? are the issue types I deleted):

As a result, those workflows are still marked as "active" and I can not delete them. I'd really appreciate any idea on how I can un-assign those workflows in order to finally delete them as well.