Gennifer Flowers took her first step toward what she hopes will be a singing career on the Las Vegas Strip, and her repertoire this week was anything but subtle. Flowers, who nearly derailed President Clinton's bid for the White House in 1992, sang "Who's Got the Last Laugh Now?" and "Why Haven't I Heard From You?" during her performance at the Electronic Retailing Association's annual conference at the MGM Grand Hotel. Flowers declined to comment on Clinton's affair with former White House intern Monica Lewinsky because, she said, she has no interest in generating any more publicity on the issue. "They don't need any of my help on that."