The chief executive of the Hearst Corporation, Victor F. Ganzi, is leaving the privately held publishing empire after nearly 30 years because of irreconcilable differences with its board of trustees over the company’s future, Reuters is reporting. Those differences weren’t disclosed by the privately held company which owns the Chronicle. Former chief executive Frank Bennack Jr., 75, who stepped down in 2002, will reassume his role as chief executive while a search committee looks for a successor. Hearst is one of the nation’s largest magazine publishers and it owns a piece of cable networks such as ESPN, A&E and Lifetime, and a controlling interest in a group of TV stations. Its newspapers include the Houston Chronicle, The Times Union in Albany, N.Y., and the Seattle Post-Intelligencer.