Stephen Colbert and David Letterman blew off some network steam and smashed some network stuff last night in a funny and oddly poignant call-back to one of Letterman’s signature late-night bits: Throwing things from the roof of the Ed Sullivan Theater. With the CBS eye logo providing a helpful sidewalk target, the two Late Show hosts tossed – or, rather, supervised the stagehand tossing – some studio chairs (it only took two before the target was smashed, releasing lots of colorful balls into the street) and, inevitably watermelons, the dropping of which was one of the favorite stunts from the old Late Show with David Letterman years. The pair finished up with Colbert sending a congratulatory cake from the network – a multi-layered wedding-style beast – sailing downward.…