QThreadData is leaked in main thread

A QThreadData object leak of the main thread(GUI thread) has been introduced in change https://codereview.qt-project.org/#change,45187
Run the following program in VS2010, you can see the leak report

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QWidget w;
    w.show();

    int ret = a.exec();

    // turn on memory leak detection
    int nCrtDbgFlag = _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG);
    nCrtDbgFlag |= _CRTDBG_LEAK_CHECK_DF;
    _CrtSetDbgFlag(nCrtDbgFlag);

    return ret;
}

The removed lines in src/corelib/kernel/qcoreapplication.cpp caused this leak, it prevent the refcount of QThreadData from being dropped to zero on app exit.