Citigroup's Weill to Step Down as CEO Wed Jul 16, 4:44 PM ET By Jonathan Stempel NEW YORK (Reuters) - Sanford Weill on Wednesday said he will step down as Citigroup Inc.'s (NYSE:C - news) chief executive, relinquishing power to a longtime lieutenant after building the world's No. 1 financial services company. Ending years of speculation about Weill's successor, Citigroup said Charles Prince, 53, will become CEO by Jan. 1. He has run Citigroup's corporate and investment bank since September, and had been the company's top lawyer. Weill, known as "Sandy," turned 70 in March and will remain chairman until...