Stephen K. Bannon is stepping down from his post as executive chairman of Breitbart News, the company announced Tuesday. Mr.  Bannon’s departure, which was forced by a onetime financial patron, Rebekah Mercer, comes as Mr. Bannon remained unable to quell the furor  over remarks attributed to him in a new book in which he questions President Trump’s mental fitness and disparages his elder son, Donald Trump Jr.