fix(test): remove not used polyfill

Author: klarkc

template/gulp-tasks/test.js

@@ -14,10 +14,7 @@ gulp.task('test:server', () => {
 
 gulp.task('test:client', (done) => {
   new Server({
-    files: [
-      'node_modules/babel-polyfill/dist/polyfill.js',
-      path.resolve(dirs.testClient, '**/*.test.js'),
-    ],
+    files: [path.resolve(dirs.testClient, '**/*.test.js')],
     configFile: path.resolve(dirs.test, 'karma.conf.js'),
     singleRun: true,
   }, done).start();