When you’re working on a workflow that contains task items or task processes from SharePoint Designer, SharePoint will create content types for these tasks. If you’ve been experimenting with several variations, you could end up with some left over content types for tasks that you’re no longer using. Of course, we’d want to keep this list clean. However, when you try to delete these unused content types, you’ll receive a “Content type is still in use” message.
This message indicates that a list or library is still using the content type. This will be the task list assigned to your workflow (usually “Workflow Tasks”). To remove the content type from the SharePoint site, you should first remove the content type (assignment) from all task-based lists. Go to the list settings and remove the content type.
You can now remove the content type from the content types gallery.