IOWA CITY — In hopes of easing toward retirement, the University of Iowa’s long-serving Graduate College Dean and Associate Provost John Keller on Thursday announced he’s stepping down July 31 — although he’ll remain on faculty as a tenured professor and as a “special assistant to the provost.”Keller — who has been UI associate provost for graduate and professional education and Graduate College dean since 2002, after stepping in as interim for two years — also will continue to co-chair the committee tasked with leading the university’s search for a new president.