Remove use of polyfill.io due to supply chain attack

[jemus42]

See https://simonwillison.net/2024/Jun/25/polyfill-supply-chain-attack/

The domain apparently serves malicious JS intermittently.

[jemus42]

I've removed the offending include but I assume that functionality needs to be pulled in from somewhere else now.
Then again, from the above blogpost:

He now works for Fastly, which started offering a free polyfill-fastly.io alternative in February. Andrew says you probably don't need that either, given that modern browsers have much better compatibility than when the service was first introduced over a decade ago.

so maybe this is just fine now.