Remove deprecated jquery.mwExtension module

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.

Change-Id: Ib66d1844b4fb8d7185b0e6607b9f98c1be632bb5