The extortion trial of City Hall officials Kenneth Brissette and Timothy Sullivan has been canceled as a federal judge decides whether to dismiss the case.Prosecutors told Judge Leo T. Sorokin yesterday they cannot prove their case based on what they charge is his misinterpretation of Hobbs Act extortion.The defense promptly responded with a motion to dismiss, which the government said it would not fight.Prosecutors may yet try to salvage their case by appealing Sorokin’s ruling that he would not reconsider his interpretation of the law.