Remove m_allowCrossOriginRequests which is no longer used.

The code using it has been removed by this change.
http://src.chromium.org/viewvc/blink/trunk/Source/core/xml/XMLHttpRequest.cpp?revision=152712

Review URL: https://chromiumcodereview.appspot.com/23890038

git-svn-id: svn://svn.chromium.org/blink/trunk@157987 bbb929c8-8fbe-4397-9dbb-9b2b20218538