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.
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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.
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