[strong] Remove all remainders of strong mode

test/mjsunit/strong/mutually-recursive-funcs.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: --strong-mode
-
-"use strong";
-
-function foo(param, fooCount, barCount) {
-  if (param === 0)
-    return {'foo': fooCount, 'bar': barCount};
-  return bar(param - 1, fooCount + 1, barCount);
-}
-
-function bar(param, fooCount, barCount) {
-  if (param === 0)
-    return {'foo': fooCount, 'bar': barCount};
-  return foo(param - 1, fooCount, barCount + 1);
-}
-
-(function TestMutuallyRecursiveFunctions() {
-  let obj = foo(10, 0, 0);
-  assertEquals(obj.foo, 5);
-  assertEquals(obj.bar, 5);
-})();