EAST RUTHERFORD, N.J. — Mike Shanahan was soaked in both rainwater and disappointment as he walked off the field Sunday for what is expected to be the final time as coach of the Washington Redskins. He had stood on the sideline absorbing a steady rain on a gray afternoon in a mostly empty, lifeless stadium as the Redskins lost their season finale to the New York Giants, 20-6, to drop their record to 3-13 with their eighth straight defeat.