Remove unused vars

Code cleanup for [JSHint support](https://trello.com/c/SQhvGHri/498-jshint-linting-support)

* Removes unused vars (reported by JSHint)
* Adjusts `.jshintrc` config: Remove obsolete prefs, add `unused: "vars"` (... will change our Travis results)
* Remove unused `/* globals */` (reported by JSHint)
* Make `ExtensionLoader.js` pass JSLint