Remove --harmony-async-await runtime flag

test/mjsunit/es8/async-function-stacktrace.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
-
 async function test(func, funcs) {
   try {
     await func();
     throw new Error("Expected " + func.toString() + " to throw");
   } catch (e) {
     var stack = e.stack.split('\n').
         slice(1).
         map(line => line.trim()).
         map(line => line.match(/at (?:(.*) )?.*$/)[1]).
         filter(x => typeof x === 'string' && x.length);
@@ skipping 152 matching lines @@
     } catch (e) {
       throw new Error("FAIL");
     }
   }, ["test"]);
 }
 
 runTests().catch(e => {
   print(e);
   quit(1);
 });