Take it off! Twilight hunk Kellan Lutz holds no illusions about the nature of his stardom. "Producers and studios know what sells," the impressively fit actor told UsMagazine.com during a Monday phone interview. "It's nice to be one of the guys that can help sell a movie by taking his shirt off." Lutz, 25, recently topped Men's Health magazine's inaugural list of Best Summer Bodies -- beating out…