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The producer of the musical “Les Miserables” is negotiating to build a wall to block off the last six rows of seats at the Shubert Theater, thereby eliminating about 25O seats and $35,000 a week in potential ticket sales. “I think it will make the Shubert a better-sized house for our show,” said producer Cameron Mackintosh, speaking from his New York office. He acknowledged that eliminating seats to a hit show is a highly unusual and perhaps unprecedented move, but added that “the rear mezzanine is starting not to sell, especially on weekdays. It will only get worse after Thanksgiving.” Bernard Jacobs, president of the Shubert Organization, which operates the 1,829-seat theater, indicated Mackintosh might get his way. “We are not in the business of offending producers,” Jacobs said. A local production of another one of Mackintosh’s hits, “Phantom of the Opera,” is scheduled to begin performances at the 2,071-set Ahmanson Theatre in May, 1989. He said he does not envision cutting back the Ahmanson’s capacity.