[strong] Remove mjsunit/strong/use-strong-flag.js

It would force us to make mjsunit.js strong-compatible, and that's too much for
now (for the upcoming features) and would have a ripple effect on other
tests. Especially debug-script.js which unships harmony features would break.

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/930293002

Cr-Commit-Position: refs/heads/master@{#26694}

test/mjsunit/strong/use-strong-flag.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: --use-strong
-
-// Check that d8 still works with the flag.
-
-// ...and that the flag is active.
-// TODO(rossberg): use something that doesn't require eval.
-assertThrows("0 == 0", SyntaxError);