It didn’t take long for the latest flat-tax advocates to start sounding like Navin Johnson, the title character in Steve Martin’s film “The Jerk.’’ Confronted with the tangled web of America’s tax code, Republican presidential candidates instinctively shout, “I don’t need any of this!’’ Just as Johnson struggled to walk away from his worldly possessions (“. . . except this ashtray . . . and this paddle game . . . and this remote control . . .’’) their resolve slowly crumbles.