JEAN SHEPHERD, WOR New York after-midnight D. J. who sold an unwritten book so well it subsequently had to be written (see story page 106), was fired in mid-program early Friday morning for attempting to sell genuine soap product - that hadn't bought time. In apparent effort to prove his program isn't too highbrow to sell things, he started urging listeners to go out and buy Sweetheart soap. After several of these free "commercials" he was yanked about 2:30 a.m. Friday and replaced with program of recorded music. WOR spokesman said station felt Mr. Shepherd was "talented and erudite" but appealed to limited audience; that his time period henceforth would be filled by Long John program of music, interviews, and stories.