Cops looking to interview a pair of ex-cons who may have witnessed two Brooklyn homicides were beaten to the punch yesterday by mystery assassins who blew the men away in the upscale Flatiron District, authorities said. Rashawn Washington, 26, and Terrance Serrano, 22, were whacked execution-style as they sat in a silver Mercury SUV at Broadway and East 19th Street just after they left the Roam Lounge, a nearby club. The killers pulled up behind the SUV in a dark sedan at 3:40 a.m. The two gunmen jumped out, drew their weapons and opened fire. Brigitte Stelzer EXECUTED: Cops remove the bodies of Rashawn Washington and Terrance Serrano from an SUV on 19th Street and Broadway, where they were shot dead yesterday. A photo of one of the victims, Rashawn Washington At least five bullets were fired at Washington and Serrano before the killers hopped back into their car and sped off, police said. Police Commissioner Ray Kelly said...