Gangster James “Whitey” Bulger’s decision not to take the witness stand in his own defense two years ago could ultimately mute any shot he has at a new trial, a panel of federal appellate justices suggested yesterday.The packed 30-minute hearing in the U.S. Court of Appeals for the First Circuit in South Boston was the 85-year-old killer’s first attempt to overturn two life sentences based on his persistent claim that he was granted immunity by the Department of Justice to maim and murder at will without fear of prosecution — a point the feds refute.