[strong] Remove all remainders of strong mode

test/mjsunit/strong/var-let-const.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: --strong-mode
-
-(function NoVar() {
-  assertThrows("'use strong'; var x = 0;", SyntaxError);
-  assertThrows("'use strong'; for(var i = 0; i < 10; ++i) { };", SyntaxError);
-})();
-
-
-(function LetIsOkay() {
-  assertTrue(eval("'use strong'; let x = 0; x === 0;"));
-  assertTrue(eval("'use strong'; for(let i = 0; i < 10; ++i) { } 0 === 0;"));
-})();
-
-
-(function ConstIsOkay() {
-  assertTrue(eval("'use strong'; const x = 0; x === 0;"));
-  assertTrue(eval("'use strong'; for(const i = 0; false;) { } 0 === 0;"));
-})();