Mayor Rahm Emanuel fired the city’s police superintendent today, a week after the release of a dash-cam video that showed a white Chicago officer fatally shooting a black teenager 16 times.Emanuel called a news conference to announce the dismissal of Garry McCarthy, who only days ago insisted to reporters that the mayor had his “back.”The mayor praised McCarthy’s leadership of the force but called it an “undeniable fact” that the public’s trust in the police had eroded.“Now …