Zachary T. Fardon stepped down as Chicago’s top federal prosecutor on Monday, 3½ years after he took the post and three days after the Trump administration abruptly ordered 46 U.S. attorneys to hand in their resignations.Fardon and the 45 others who were fired were holdovers from President Barack Obama’s administration.Fardon’s first assistant, Joel R. Levin, will serve as acting U.S. attorney for the Northern District of Illinois until a permanent successor is in place.U.S. attorneys are appointed …