Remove flags for ES2015 features shipped in M48

test/mjsunit/es6/array-concat.js

 // Copyright 2014 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-concat-spreadable --harmony-proxies --harmony-reflect
+// Flags: --harmony-proxies --harmony-reflect
 
 (function testArrayConcatArity() {
   "use strict";
   assertEquals(1, Array.prototype.concat.length);
 })();
 
 
 (function testArrayConcatNoPrototype() {
   "use strict";
   assertEquals(void 0, Array.prototype.concat.prototype);
@@ skipping 849 matching lines @@
     if (key === "length") return Symbol();
     if (key === "2") return "baz";
     if (key === "3") return "bar";
   };
   var target = [];
   var obj = new Proxy(target, {get: getTrap, has: () => true});
 
   assertThrows(() => [].concat(obj), TypeError);
   assertThrows(() => Array.prototype.concat.apply(obj), TypeError);
 })();