The “Late Show” hosts reprised a classic bit before a F-bomb ending. Late-night legend David Letterman has made no secret of his contempt for CBS’ decision to cancel “The Late Show,” the program he hosted from 1993 until he retired in 2015. And on Thursday, during one of the final episodes of Stephen Colbert’s tenure before the show is taken off the air for good on May 21, Letterman delivered an R-rated message to the network. Colbert and Letterman revived a classic bit from Letterman’s era, in