WASHINGTON — Democrats on the Senate Judiciary Committee called on U.S. Supreme Court Chief Justice John Roberts on Monday to open an investigation into the undisclosed acceptance of luxury trips taken by Justice Clarence Thomas and his wife that were paid for by a Republican megadonor.The letter said the committee plans to hold a hearing in coming days regarding the “need to restore confidence in the Supreme Court’s ethical standards.” And if the Supreme Court does not deal with the issue on its …