WASHINGTON—General Motors Co. said on Thursday that Dan Ammann, the chief executive of its majority-owned Cruise self-driving car subsidiary, is leaving the company, effective immediately. The U.S. automaker did not give a reason for the departure of Ammann, a former GM president and chief financial officer. GM shares fell 2.7 percent in after hours trading following the announcement. GM said Kyle Vogt, Cruise president and chief technical officer, will serve as interim CEO. In addition, Wesley Bush, the former chairman and CEO of Northrop Grumman and a GM board member, will join the Cruise board, it said. Cruise had once hoped to start its commercial robo-taxi service by the end of 2019. Two years later, it is still waiting for final permits to start a limited service in a section of San Francisco. GM said on Thursday it would accelerate its strategy in which Cruise will play an integral role …