code: func (s *AuthorizationsService) DeleteImpersonation(ctx context.Context, username string) (*Response, error) {
  	u := fmt.Sprintf("admin/users/%v/authorizations", username)
  	req, err := s.client.NewRequest("DELETE", u, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // DeleteImpersonation deletes an impersonation OAuth token.
  //
  // NOTE: there can be only one at a time.
  //
  // GitHub API docs: https://developer.github.com/enterprise/v3/enterprise-admin/users/#delete-an-impersonation-oauth-token