Taquería El Califa de León, the modest Mexico City taquería that made history as the only taco stand with a Michelin star, has lost that coveted recognition two years after earning it. The latest edition of the Michelin Guide Mexico gives no reason for the downgrade. But it has said in the past that “restaurants can lose their stars if they close during the year of assessment, or if they do not maintain their standards to make it into the next e…