mock_u2f_device.js: remove IIFE

spec/javascripts/u2f/mock_u2f_device.js

-/* eslint-disable no-unused-expressions, no-return-assign, no-param-reassign */
+/* eslint-disable no-unused-expressions */
 
 export default class MockU2FDevice {
   constructor() {
     this.respondToAuthenticateRequest = this.respondToAuthenticateRequest.bind(this);
     this.respondToRegisterRequest = this.respondToRegisterRequest.bind(this);
     window.u2f || (window.u2f = {});
-    window.u2f.register = (function(_this) {
-      return function(appId, registerRequests, signRequests, callback) {
-        return (_this.registerCallback = callback);
-      };
-    })(this);
-    window.u2f.sign = (function(_this) {
-      return function(appId, challenges, signRequests, callback) {
-        return (_this.authenticateCallback = callback);
-      };
-    })(this);
+    window.u2f.register = (appId, registerRequests, signRequests, callback) => {
+      this.registerCallback = callback;
+    };
+    window.u2f.sign = (appId, challenges, signRequests, callback) => {
+      this.authenticateCallback = callback;
+    };
   }
 
   respondToRegisterRequest(params) {