LONDON — Prince William took a day out from royal duties on Friday to serve as an usher at the wedding of his friend Hugh Grosvenor, the Duke of Westminster. The wedding of 33-year-old Grosvenor, one of Britain’s wealthiest landowners, to his partner Olivia Henson, 31, is one of the high-society events of the year in Britain. [time-brightcove not-tgx=”true”] William, 41, arrived with his fellow ushers at Chester Cathedral in northwest England ahead of the midday service.