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