The Stiegelbauer workshop, where Stephen DeMaria coordinated the construction of sets for “Saturday Night Live,” is in the Brooklyn Navy Yard, miles from 30 Rockefeller Plaza. Geographically, and in the minds of “S.N.L.” fans, DeMaria’s name was always far removed from the prestige and glamour of the show that has defined American comedy since 1975. But while most viewers have never heard of him, he has spent 50 years setting the scenes for the show’s most memorable moments. And now he is done.