NEWARK—For years, a savage murderer called the “Torso Killer” dribbled out confessions to a New Jersey detective, closing cold cases into the brutal deaths of four young women that dated back to the late 1960s. But there was one unsolved case the notorious serial killer would never admit to: The slayings of 17-year-old Mary Ann Pryor and 16-year-old Lorraine Marie Kelly in 1974. On Tuesday, Richard Cottingham, 74, sat in a wheelchair in beige prison scrubs as he pleaded guilty to kidnapping the teenage girls and raping them for days before he drowned them in a motel room bathtub. The teenagers were last seen on Aug. 9, 1974, in North Bergen. They’d left home for a trip 13 miles north to a Paramus mall, telling family they planned to take a bus there to buy bathing suits for a trip to the Jersey Shore. Witnesses at the time told police …