Resetting your workflow to default will delete most of your settings as well as Smart Lists.
To do that, go to ☰→Workflow Library→Import/Export and then click Delete Workflow which will delete most of your settings.
Once in Workflow Library, under Import/Export you can Delete workflow.
Deleted workflows are gone forever, so make sure you save a setup first if you might want to use it again in the future.
If you also want to delete all your content read this article.