[strong] Remove a regression test for "use before var".

test/mjsunit/strong/use-before-var.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
-
-function strongFunction() {
-  "use strong";
-  // This should not be a "use before declaration" error, even though the
-  // variable is defined later (by non-strong code). Rationale: Non-strong code
-  // is allowed to use var declarations, and the semantics of var declarations
-  // say that the variable should be usable before the declaration.
-  return notStrong + 123;
-}
-
-var notStrong = 456;
-
-(function TestStrongFunctionUsingLaterDefinedNonStrongVar() {
-  assertEquals(strongFunction(), 579);
-})();