Forget plummeting poll numbers, John Kerry's got a much bigger PR problem. The junior senator's been dropped from the menu at Bartley's Burger Cottage in Harvard Square. Bill Bartley, son of the famed burger joint's founder, Joe Bartley, says he wants a menu with more sizzle, and that means Barack Obama is in and Kerry is out. "How long has he been my senator, and what's he ever done except embarrass me?" Bartley said yesterday.