BOSTON/NEW YORK (Reuters) – An official for the second-largest U.S. public pension system on Thursday urged Viacom Inc to do away with its dual-class share structure, the latest call from an investor for change at the media empire controlled by the aging mogul Sumner Redstone as doubts grow over his ability to lead. Redstone, 92 years old, controls about 80 percent of the voting rights at Viacom, and the company has a second class of shares with no voting rights. The California State Teachers’ Retirement System has 1.1 million non-voting shares.