A deranged gunman shot and killed a 48-year-old straphanger aboard a Manhattan subway train Sunday morning in what cops are calling an unprovoked attack. The shooter, who was still on the loose Sunday afternoon, paced back and forth in the last car of the northbound Q train around 11:42 a.m. before he opened fire on the random victim seated inside, police and sources said. “Completely random,” one police source said. The victim, whose name wasn’t immediately released, was rushed to Bellevue Hospital where he was later pronounced dead, police said. According to witnesses, the suspect was walking back and forth...