BUCHAREST, July 15 (Reuters) – A Black Hawk helicopter belonging to the U.S. military made an emergency landing on a Bucharest street on Thursday, tearing down two public street lights and stopping traffic but making no casualties. The helicopter was part of a group of six which were rehearsing ahead of events to mark Romanian Air Force day and the end of Romanian military presence in Afghanistan on July 20-21. Footage from eyewitnesses show it suddenly losing altitude and flying low above cars until landing nearby in a downtown roundabout…