David Letterman marched back into the Ed Sullivan Theater on Thursday night and unloaded on CBS over its decision to cancel Stephen Colbert’s late-night show — before helping toss couches, watermelons and a wedding cake off the roof of the landmark Manhattan venue. The former “Late Show” host joined Colbert for a blistering on-air segment that mocked the network and its new ownership just days before Colbert’s final broadcast. At the conclusion of the segment, the outgoing host asked Letterman if he wanted to offer some parting words to viewers. David Letterman returned to the Ed Sullivan Theater on Thursday night and joined Stephen Colbert in a rooftop stunt targeting CBS after the network canceled “The Late Show.” Scott Kowalchyk /CBS Letterman took the opportunity to address CBS, instead, saying: “To the folks at CBS, in the words of the great Edward R.…