Remove Math extension from Dockerfile

[Yaron Koren]

The Math extension has been bundled in with MediaWiki since version 1.38:

https://www.mediawiki.org/wiki/Extension:Math

Given that, it should no longer be downloaded by the Dockerfile, once the MediaWiki version is upgraded to 1.39.

[Yaron Koren]

Never mind! It turns out that this was already, done, here: 93e3be6

I'm closing this.