Remove --harmony-async-await runtime flag

test/mjsunit/regress/regress-5405.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --harmony-async-await --allow-natives-syntax
+// Flags: --allow-natives-syntax
 
 let log = [];
 
 (async function() {
   with ({get ['.promise']() { log.push('async') }}) {
     return 10;
   }
 })();
 %RunMicrotasks();
 
 (function() {
   with ({get ['.new.target']() { log.push('new.target') }}) {
     return new.target;
   }
 })();
 
 (function() {
   with ({get ['this']() { log.push('this') }}) {
     return this;
   }
 })();
 
 assertArrayEquals([], log);