MDL-79003 eslint: Enable 'reportUnusedDisableDirectives' eslint setting

This commit is a backport of MDL-74301 which should have been backported
at the time.

This will warn on unused /* eslint-disable */ statements.

.eslintrc

@@ -20,6 +20,8 @@
     'M': true,
     'Y': true
   },
+  // Warn about unused eslint-disable statements.
+  'reportUnusedDisableDirectives': true,
   'rules': {
     // See http://eslint.org/docs/rules/ for all rules and explanations of all
     // rules.

admin/tool/lp/amd/src/competency_rule.js

@@ -71,14 +71,12 @@ define(['jquery'], function($) {
         return null;
     };
 
-    // eslint-disable-line valid-jsdoc
     /**
      * Return the type of the module.
      *
      * @return {String}
      * @method getType
      */
-    // eslint-enable-line valid-jsdoc
     Rule.prototype.getType = function() {
         throw new Error('Not implemented');
     };