NEW YORK — A Queens man who hacked up an 81-year-old veteran in front of a little girl more than four decades ago has pleaded guilty to manslaughter charges, prosecutors said Tuesday. Martin Motta, 75, is facing 20 years in prison for the 1976 slaying of George Seitz. “For the gruesome murder of a World War I veteran, the defendant eluded arrest for more than 46 years,” Queens District Attorney Melinda Katz said. “Now he is headed to prison thanks to the collaboration between the NYPD and our Cold Case Unit.” Motta was arrested in November, two years after an anonymous caller directed police t…