code: func (s *FileStorage) DeleteSite(domain string) error {
  	err := os.Remove(s.siteCertFile(domain))
  	if err != nil {
  		if os.IsNotExist(err) {
  			return ErrNotExist(err)
  		}
  		return err
  	}
  	return nil
  }
docs: // DeleteSite implements Storage.DeleteSite by deleting just the cert from
  // disk. If it is not present, an instance of ErrNotExist is returned.