mediawiki.page.ready: Convert to packageFiles, remove jquery.checkboxShiftClick

* Remove redundant closures around the two files.

* Export checkboxShiftClick as a regular function.
  This jQuery plugin is not used anywhere in Wikimedia Gerrit,
  nor elsewhere indexed by Codesearch, nor anywhere on-wiki
  in User, Project or MediaWiki namespaces.
  As such, remove the deprecated module alias as well.

Change-Id: I6bc41036829964080abf1ab0bbd306356bb8fe57