Remove dependency on Polyfill

[aspiwack]

It appears that the documentation uses an extension hosted in the polyfill.io domain. This domain has apparently been compromised https://www.sonatype.com/blog/polyfill.io-supply-chain-attack-hits-100000-websites-all-you-need-to-know delivering malicious javascript to its client.

Let's not wait.

[turion]

CC @reubenharry The only reference to polyfill I could find is in docs/mkdocs.yml Do you know what it does, and how we can remove that?

[turion]

From what I read in https://developer.mozilla.org/en-US/docs/Glossary/Polyfill we should be safe to simply remove the one line referencing polyfill. The website will stop working as intended on older browsers, but that's the whole downside.

[aspiwack]

Then let's remove for now, and we can think later about whether we want to regain the functionality.

[aspiwack]

(Immediate mitigation done in #349 )

[reubenharry]

Don't remember/know. Pretty sure it's ok to remove.

[aspiwack]

Ok, then, let's leave it like this, and if we want it again, we'll open a new issue/PR/thing.

Thanks both.