A workflow remains activated when the only project using it is deleted

Context :
I copied an existing, active workflow to add a 6th step. I added the new status, etc.
I created a new workflow scheme and a new project so as to test the new workflow. I was not satisfied with the screens on the introduced transitions between steps.

Action:
As I was unable to edit the workflow because it was active, I deleted my test project.

Result:
Unfortunately, the workflow is still reported as active, and therefore I still cannot edit it.

Comment:
I also deleted the test workflow scheme I created, but the workflow remains active.
I guess the workaround is to copy this workflow and work on the copy, but I fear I will have to keep this "ghost workflow".