refactor: enable ESLint rule "no-unused-vars" and handle related issues (#4080)

In PR #4072 GitHub Bot complained about an unused var. Instead of just
removing that one, I checked why ESLint hadn't complained about it: We
had disabled the rule for it.

So I enabled rule and resolved the issues that ESLint then detected.

Related to #4073

js/loader.js

@@ -1,5 +1,6 @@
 /* global defaultModules, vendor */
 
+// eslint-disable-next-line no-unused-vars
 const Loader = (function () {
 
 	/* Create helper variables */