Billionaire investor William Ackman said on Thursday he sold about 5 million shares in drug company Valeant International in order to generate a tax loss as investors in his Pershing Square Capital Management face the biggest loss in the company’s history. Ackman’s hedge fund now owns 29.1 million shares in Valeant, or 8.5 percent of the company, down from 34.1 million shares, or 9.9 percent, according to a regulatory filing made after the market closed on Thursday, the last trading day of 2015.