Remove invalid tests for %RegExpPrototype%

From ECMA262, section 16:

>  Except as restricted in 16.2, an implementation may provide
>  additional types, values, objects, properties, and functions beyond
>  those described in this specification.

Section 16.2 makes no mention of the `lastIndex` property of the
%RegExpPrototype% intrinsic. It is therefor not a violation to define
%such an "own" property.

Remove the tests that assert otherwise.

test/built-ins/RegExp/prototype/lastIndex/15.10.7.5-1.js

-// Copyright (c) 2012 Ecma International.  All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-
-/*---
-es5id: 15.10.7.5-1
-description: RegExp.prototype.lastIndex is of type Undefined
----*/
-
-assert.sameValue(typeof(RegExp.prototype.lastIndex), 'undefined', 'typeof(RegExp.prototype.lastIndex)');

test/built-ins/RegExp/prototype/lastIndex/15.10.7.5-2.js

-// Copyright (c) 2012 Ecma International.  All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-
-/*---
-es5id: 15.10.7.5-2
-description: RegExp.prototype.lastIndex is not present
----*/
-
-  var d = Object.getOwnPropertyDescriptor(RegExp.prototype, 'lastIndex');
-
-assert.sameValue(d, undefined, 'd');