A little over a month after a democratic socialist became mayor of New York City, billionaire Ken Griffin and his finance companies, Citadel and Citadel Securities, are voting with their feet. The firms’ old Park Avenue offices are scheduled to be demolished this spring, but rather than walk out on the Big Apple, as Griffin did in Chicago, the hedge funder wants to build the city’s next trophy office building — a 1.9 million-square-foot glass and steel tower at 350 Park Avenue.