Remove flags for ES2015 features shipped in M48

test/mjsunit/regress/regress-crbug-516775.js

 // Copyright 2015 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: --allow-natives-syntax --harmony-concat-spreadable
+// Flags: --allow-natives-syntax
 
 function arguments_with_length_getter(f) {
   arguments.__defineGetter__('length', f);
   return arguments;
 }
 
 var count = 0;
 function increment_count_return() { count++; return "boom"; }
 function increment_count_throw() { count++; throw "boom"; }
 
@@ skipping 28 matching lines @@
 a5.__proto__ = arguments_with_length_getter(increment_count_return);
 a5[Symbol.isConcatSpreadable] = true;
 [].concat(a5);
 assertEquals(3, count);
 
 var a6 = {};
 a6.__proto__ = arguments_with_length_getter(increment_count_throw);
 a6[Symbol.isConcatSpreadable] = true;
 assertThrows(function() { [].concat(a6); });
 assertEquals(4, count);