code: func (s *ProjectsService) RemoveProjectCollaborator(ctx context.Context, id int64, username string) (*Response, error) {
  	u := fmt.Sprintf("projects/%v/collaborators/%v", id, username)
  	req, err := s.client.NewRequest("DELETE", u, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	// TODO: remove custom Accept header when this API fully launches.
  	req.Header.Set("Accept", mediaTypeProjectsPreview)
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // RemoveProjectCollaborator removes a collaborator from an organization project.
  // You must be an organization owner or a project admin to remove a collaborator.
  //
  // GitHub API docs: https://developer.github.com/v3/projects/collaborators/#remove-user-as-a-collaborator