SAN DIEGO — A 20-year-old killer whale that has been at SeaWorld San Diego since his birth died Thursday night following treatment for an infection, the park announced Friday. Despite aggressive efforts to treat the infection, the orca, Nakai, died surrounded by members of the animal care and medical teams who had worked closely with him over the last two decades, SeaWorld San Diego said in a statement. Nakai’s death leaves SeaWorld with eight remaining killer whales, ranging in age from 9 to 57. The marine park is unable to expand its orca population after having instituted a breeding ban in …