Venerable Tony Cheng’s Chinese restaurant, that Mount Vernon square mainstay, is gone.  Now occupying the regal rooms of this grand townhouse is El Patron, serving Mexican. The cavernous dining rooms have been given a coat of sunny yellow paint, their spareness offset with a few hanging portraits. Expect simple south-of-the-border classics such as fajitas, burritos, tacos and grilled meats. Tortas (sandwiches on ciabatta bread) round out the menu, along with house specialties like pasta poblana with shrimp, chicken mole poblano and steak de la casa. Traditional Mexican desserts— flan and tres leches cake— end the meal.  Open for lunch Tuesday through Saturday; dinner only on Sunday; closed Monday.  801 N. Charles St., 410-244-8494