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
Done (https://github.com/MarcelGerber/brackets/commit/5c44c876c30a1ef9965b3a9db53165462cb65fc2).
I didn't add it back to `SidebarView` as it never uses `brackets`.