If the set of “ER” is devoid of the usual holiday cheer, it’s because the cast is about to film the episode that will mark the exit of George Clooney. After five seasons, last years’ Sexiest Man Alive will end his run as pediatrics specialist Doug Ross in an episode that will air during the February sweeps. Sources say Clooney’s character will not be killed off, and there’s every possibility that he could reemerge in an episode next season or thereafter. Clooney will not be replaced, but Noah Wyle is expected to subtly assume the heartthrob slot on the show. Other cast members will get a boost in their screen time. Clooney is leaving to concentrate on his film career and his WB-based TV- and movie-production companies.