American Football: President Donald Trump says National Football League owners should fire players who kneel during the national anthem. And he’s encouraging spectators to walk out in protest. In an extended riff during a freewheeling rally speech in Alabama yesterday, Trump also bemoaned that football games have become less violent. ‘‘They’re ruining the game,’’ he complained. Several athletes, including a handful of NFL players, have refused to stand during The Star-Spangled Banner to protest of the treatment of blacks by police. Quarterback Colin Kaepernick, who started the trend last year when he played for the San Francisco 49ers, hasn’t been signed by an NFL team for this season.