Go to any acting class and chances are at least some of the hopefuls there will tell you, "I don't want to be a star, I just want to work." That simple dream doesn't come true for many, but it surely has for William Schallert. He has never been a "star" nor rarely seen his name above the title, yet he seems to have worked continuously since his screen debut in "Lonely Hearts Bandits" (1950). Tall and wiry, with hair that seemed stuck in the receding hairline phase, he could portray mild-mannered librarians ("Leave It to Beaver"), kindly fathers ("The Patty Duke Show"), or basically good people