Check out WP Offload S3. There’s an option to delete local files after they’re copied to S3. There is a Lite version, but you’ll need the Pro version to copy existing media files over.
S3 also isn’t a CDN (or at least, not supposed to be), but the plugin will help you configure S3’d images to be served over Cloudfront, which is a CDN.
Although if the only concern is to free up disk space, Cloudfront wouldn’t be necessary.