remove polyfill.io references

[pkra]

The domain is apparently being acquired and users are already experiencing problems https://fosstodon.org/@haubles/111995201660738714

[dpvc]

Thanks, Peter. I will definitely change that, and make an announcement on the forum lists.

[pkra]

Some more background in josdejong/mathjs#3167

[mhils]

This should probably be expedited in light of https://sansec.io/research/polyfill-supply-chain-attack. https://www.mathjax.org/#gettingstarted still references polyfill.io.

[dpvc]

I have updated the web site to remove polyfill.io, to give a warning about its use, and to link to the article cited.