A mail carrier failed to show the U.S. Postal Service violated the Americans with Disabilities Act when it did not assign him light-duty work after he injured his foot, a federal judge held.In a written opinion this week, U.S. District Judge Charles R. Norgle Sr. also held Jose L. Vargas failed to show the postal service’s decision not to assign him work was motivated by racial discrimination.Vargas presented no evidence that any other postal worker — of any race — who was placed on light-duty status was …