Folk music icon Joni Mitchell’s stirring performance of her classic hit Both Sides Now brought down the house at this year’s Grammy Awards, but sources claim it was really her final farewell, RadarOnline.com has learned. According to a sensational report, insiders claim Mitchell, 80, delivered a memorable performance with her fans — and mortality — in mind. “Joni knows she doesn’t have much time left, and she wanted to give her fans a last chance to see her singing one of her most significant songs,” an insider told the National Enquirer. Just getting on stage again was said to be a long shot …