Casper, the online mattress retailer, has joined rival Leesa, as well as Cars.com and Peloton in pulling commercials from Sean Hannity’s show on Fox News after the host continued to push a debunked story that a former Democratic National Committee staffer, Seth Rich, had been murdered to keep him from leaking information to WikiLeaks. Mr. Hannity has continued to push his claims even after Mr. Rich’s family appealed for him to stop and Fox News retracted its reporting on the fake news story.