Revert "Turn old space cons strings into regular external strings (not short)."

test/mjsunit/regress/regress-sliced-external-cons-regexp.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: --expose-externalize-string --expose-gc
-
-var re = /(B)/;
-var cons1 = "0123456789" + "ABCDEFGHIJ";
-var cons2 = "0123456789\u1234" + "ABCDEFGHIJ";
-gc();
-gc();  // Promote cons.
-
-try { externalizeString(cons1, false); } catch (e) { }
-try { externalizeString(cons2, true); } catch (e) { }
-
-var slice1 = cons1.slice(1,-1);
-var slice2 = cons2.slice(1,-1);
-for (var i = 0; i < 10; i++) {
-  assertEquals(["B", "B"], re.exec(slice1));
-  assertEquals(["B", "B"], re.exec(slice2));
-}