Lou Dobbs, one of former President Donald J. Trump’s most loyal media supporters, abruptly lost his pulpit on Friday when Fox Business canceled his weekday television show, which had become a frequent clearinghouse for baseless theories of electoral fraud in the weeks after Mr. Trump lost the 2020 presidential race.Mr. Dobbs’s decade-long tenure at the network ended with little warning — a guest host filled in for his Friday slot — only a day after the election technology company Smartmatic filed a defamation lawsuit against Rupert Murdoch’s Fox Corporation and Fox News.The suit, which seeks damages of at …