(Reuters) - A judge has rejected a proposed severance package of nearly $20 million for Thomas Horton, the chief executive of American Airlines parent company AMR Corp, saying the payout wasn't allowed under federal bankruptcy law. Bankruptcy Judge Sean Lane in Manhattan issued his ruling on Thursday. The judge had approved AMR's plan to merge with US Airways Group Inc on March 27, a step toward creating the world's largest airline. The combined company is to be run by US Airways Chief Executive Doug Parker, with Horton serving as chairman. ...