In 2008, when “Step Brothers” was released, Will Ferrell was 41 and Mary Steenburgen was 55. In the film, she plays his mom. I bet that hurt when her agent called. I bet it also hurt when she read the script and counted how many F-bombs she would be dropping for the sake of a cheap laugh. What was that Mary? You won an Oscar? That’s cool, but were you ever a “mystery slimed celebrity” at the Nickelodeon Kids Choice Awards? This plotless movie is a loosely strung sequence of admittedly funny gags, but Ferrell’s shock-and-awe comedy style grows old.