As of July 7, 2020, you can now delete the results of individual workflow runs. To do this, navigate to your workflow, find the workflow run that you want to delete, and select the “…” menu. In this menu, select “Delete workflow run”.
The workflow run and its logs will be removed.
Currently, you must do this for each workflow run individually.
edit: As of 2021 Feb it seems that after all workflow runs are deleted
the workflow it self disappears. One comment below also seems to confirm this.