A federal judge has declined to certify a class in a negligence case against the largest football helmet manufacturer in the world.U.S. District Judge Matthew F. Kennelly ruled potentially hundreds of thousands of plaintiffs cannot sue Riddell in a single action, citing the vast number of likely differences in their claims.The proposed class of former college and high school football players who wore Riddell’s helmets between 1975, when it became the market leader and an iconic brand using NFL and NCAA partnerships …