If you want to delete a workout from your list of completed or planned workouts, just swipe left on the workout you wanrt to delete, and a big red delete button will appear:
Please note that this will not delete this workout’s name from the list of workout names you’re presented when beginning a new workout and giving it a name. This list always shows all workout names that are currently in your library of completed or planned workouts. In order to remove a name from there, you could bullk-rename all workouts with that name (feature included in the pro app and available as a paid feature for the free app)