Walgreens To Invade Yenching Palace? The old restaurants are dropping like flies. At the end of 2005, Truman-era steakhouse Blackie’s House of Beef served up its last porterhouse. In August, embattled real estate developer Douglas Jemal purchased the land occupied by A.V. Ristorante — a move that will force the 57-year-old Italian restaurant to close by October 2007. And yesterday, an enterprising tipster told us that the rumor on the street (well, on Connecticut Avenue)…