Remove deprecated jquery.mwExtension module
- Deprecated since MediaWiki 1.26.
- Not used anywhere in Wikimedia Git.
- Grafana mw-js-deprecate dashboard shows < 1 hit per day on average for any of the jquery.mwExtension properties, during the past 3 weeks. Most days 0, some days 18 individual hits. By comparison, legacy wikibits before we removed it in May 2017 was down to about 6,000/24h (combined), so removal is quite overdue.