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
Drive-by note: we can't just remove unused references to Globals, since this is used to ensure a load order that sets up the global `brackets.*` members early enough.  You'll have to change cases like this to a require() call that's not stored into a variable.