In any case prosecuted for violation of a municipal ordinance the village solicitor or city director of law, and for a statute, he or the prosecuting attorney, shall present the case for the municipal corporation and the state respectively, but either may delegate the responsibility to some other attorney in a proper case, or, if the defendant be unrepresented by counsel may with leave of court, withdraw from the case. But the magistrate or judge shall not permit prosecution of any criminal case by private attorney employed or retained by a complaining witness.