[strong] Remove all remainders of strong mode

test/mjsunit/strong/for-in.js

Index: test/mjsunit/strong/for-in.js
diff --git a/test/mjsunit/strong/for-in.js b/test/mjsunit/strong/for-in.js
deleted file mode 100644
index 641248c408e73412aca464f1fe6de1622e9b3e29..0000000000000000000000000000000000000000
--- a/test/mjsunit/strong/for-in.js
+++ /dev/null
@@ -1,11 +0,0 @@
-// 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: --strong-mode
-
-(function NoForInStatement() {
-  assertThrows("'use strong'; for (x in []) {}", SyntaxError);
-  assertThrows("'use strong'; for (let x in []) {}", SyntaxError);
-  assertThrows("'use strong'; for (const x in []) {}", SyntaxError);
-})();