Unregister useless checkers to speed up twistedchecker.

        @param allowedMessages: output messages allowed in twistedchecker

def restrictCheckers(self, allowedMessages):
        """
        Unregister useless checkers to speed up twistedchecker.

        @param allowedMessages: output messages allowed in twistedchecker
        """
        uselessCheckers = self.findUselessCheckers(allowedMessages)
        # Unregister these checkers
        for checker in uselessCheckers:
            self.unregisterChecker(checker)