HONG KONG — Under the cover of darkness early Thursday, authorities in Hong Kong tore down a public sculpture dedicated to the victims of the Tiananmen Square massacre, accelerating a campaign to erase the crackdown from public recollection and stamp out dissent in a city that until recently was one of Asia’s freest. The 26-foot-tall artwork, known as the “Pillar of Shame,” had stood at the University of Hong Kong for nearly a quarter-century and honored the hundreds, if not thousands, of students and others killed on June 4, 1989, when the Chinese military crushed pro-democracy protests....