[security] Remove polyfill.io dependency in docs

[AlexRTer]

Currently, mkdocs uses polyfill.io to build the online Daphne documentation.
It has been reported that the site is involved in a supply chain attack delivering malicious code instead.

• https://blog.cloudflare.com/automatically-replacing-polyfill-io-links-with-cloudflares-mirror-for-a-safer-internet
• https://www.sonatype.com/blog/polyfill.io-supply-chain-attack-hits-100000-websites-all-you-need-to-know

This PR simply removes the dependency without replacement which should not impact the documentation in most browsers. If needed, e.g. Cloudflare currently provides polyfill over their cdn.

[pdamme]

Thanks a lot for this security fix, @AlexRTer!