The new president on “The West Wing” will be a real short-timer: NBC announced yesterday it was pulling the plug on the Emmy-winning political drama in May after seven seasons. Although this season’s story line with a presidential campaign involving a Democrat played by Jimmy Smits and Republican portrayed by Alan Alda has been strong critically, ratings have sunk with its move to Sunday nights. The decision to cancel it was made before actor John Spencer, who played former presidential chief of staff Leo McGarry, died of a heart attack Dec. 16, said Kevin Reilly, NBC entertainment president.