little more than a year after announcing that he was battling Parkinson's disease, Canadian actor Michael J. Fox bid an emotional farewell to his hit sitcom Spin City.   Spin City was Fox's return to TV after his three-time Emmy winning role as Young Republican Alex P. Keaton on Family Ties. He rode that success to a film career, starring in the Back To The Future trilogy, The American President and, most recently, as the voice of the mouse in Stuart Little.   The Edmonton-born Fox, 38, earned three straight Golden Globes for Best Actor in a TV series for his role as New York City Deputy Mayor, Mike Flaherty. Fox said his departure was spurred by wanting to spend more time with his family and working towards a cure for Parkinson's.