This wry, wisecracking blonde never became a star, but brightened up scores of films in the 1930s as the heroine's pal, co-worker or rival. The Kentucky native got her start as a model, drifting into acting in the mid-20s. By 1927, she was appearing on Broadway with Helen Hayes in "Coquette" and three years later was discovered by D.W. Griffith, who cast her as the wan, tragic Ann Rutledge in "Abraham Lincoln".But Merkel's true forte was comedy, and she was almost promptly typed as the knowing but sunny-natured second lead in films like "Private Lives" (1930), "42nd Street" (1933), "On