‘Walker Independence’ and ‘The Winchesters’ are far from pioneering prequels

If all shows were animated like “The Simpsons,” networks wouldn’t need to strain to keep them alive. Yet live-action dramas come with shelf lives, which explains the CW’s twin attempts to extend two of its franchises with prequels: “Walker Independence,” a back-to-the-Old-West adjunct to its Texas Ranger reboot; and “The Winchesters,” a one-generation-back rekindling of the “Supernatural” flame.