Mathjax CDN is expiring in a few days and they keep messing about.

They (very late) realised that when they shut down their CDN, all the users of the CDN have to mess about to get the extensions working (accessibility being the most important one, but mchem is optional, but pretty important too).

We already dealt with this in MDL-58461 by installing the accessibility extensions locally.

Because MathJax released 2.7.1 which bundles those extensions instead - it would be better for us to remove our version of those extensions and update to 2.7.1.

The benefits are that the extensions will stay compatible with different mathjax versions, one less third party library to update and mchem is easily supported by changing config (using the current solution anyone who wanted mchem would have to download and install it themselves).

See their updated notice about the cdn: https://www.mathjax.org/cdn-shutting-down/