Steven Libman, whose 17 years at the helm of the Pittsburgh Ballet Theater make him one of the longest-tenured dance executives in the U.S., is resigning from the company and will depart later this summer. Libman is credited with raising the PBT’s endowment to become the fourth-largest in the country among ballet troupes, but since 2001, the company has struggled, and several of Libman’s decisions have been questioned by members of the company and the public.