There'll be no Miracle on 34th Street for Macy's this Christmas. Executives of the world's largest department store said ''bah, humbug'' to the script for producer John Hughes' update of the 1934 classic. Why? It seems the plot in which Kris Kringle (Sir Richard Attenborough) fends off a corporate raider hits too close to home. (The store went private in a leveraged buyout in 1986.) ''It's silly for them not to want the publicity,'' says one of the film's stars, Elizabeth Perkins (The Flintstones). ''I don't understand it.'' Macy's says the store just wasn't interested in tampering with a classic. ''We felt the original could not be improved,'' says a spokeswoman. In its place Hughes has created the fictional CF Coles and Company. No word on what will sub for Gimbels.