code: def delete_category(self, category):
          
          # Remove mapping of metrics-to-category
          category_key = self._category_key(category)
          self.r.delete(category_key)
  
          # Remove category from Set
          self.r.srem(self._categories_key, category)
docs: Removes the category from Redis. This doesn't touch the metrics;
          they simply become uncategorized.