It’s lights out for a beloved holiday tradition in Manhattan. Saks Fifth Avenue’s annual holiday light show at its flagship location has been canceled after nearly two decades of yuletide spectacle — apparently because of cost-cutting, The Post has learned. “For many years, the holidays at Saks Fifth Avenue included a light show at our flagship store, and for some time, we have contemplated changing our approach,” a Saks Fifth Avenue rep told The Post on Monday. Now, instead of the glowing spectacular, “in our 100th year, which also marks the anniversary of the flagship, we are celebrating the season...