code: func (s *AppsService) RemoveRepository(ctx context.Context, instID, repoID int64) (*Response, error) {
  	u := fmt.Sprintf("user/installations/%v/repositories/%v", instID, repoID)
  	req, err := s.client.NewRequest("DELETE", u, nil)
  	if err != nil {
  		return nil, err
  	}
  	req.Header.Set("Accept", mediaTypeIntegrationPreview)
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // RemoveRepository removes a single repository from an installation.
  //
  // GitHub docs: https://developer.github.com/v3/apps/installations/#remove-repository-from-installation