Removed unused files

* select2.jquery.json - This was previously used by the jQuery Plugin
  Registry which was shut down and put into read-only mode a few
  years ago. Since nobody else appears to use this file, it doesn't
  make sense for us to keep bumping the version in it and keeping it
  up to date.
* vendor/ - This was needed back when the full version of Select2
  actually bundled its own version of jQuery. Since we never actually
  did this, we no longer need to keep the copy of jQuery around.