code: func (s *TeamsService) RemoveTeamMembership(ctx context.Context, team int64, user string) (*Response, error) {
  	u := fmt.Sprintf("teams/%v/memberships/%v", team, user)
  	req, err := s.client.NewRequest("DELETE", u, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // RemoveTeamMembership removes a user from a team.
  //
  // GitHub API docs: https://developer.github.com/v3/teams/members/#remove-team-membership