Make sure QThreadData and QAdoptedThread object is destroyed at app exit

Task-number: QTBUG-30134

Restore the lines in qcoreapplication.cpp removed by commit
950b35cf97 to ensure QThreadData and
QAdoptedThread object of main thread is destroyed at application exit.
We don't set QCoreApplicationPriavte::theMainThread to 0 as before since
it will be set to zero in QThreadData::~QThreadData()

Change-Id: I8ee56aff5a933ce1d812b07fb00a29ed0839ab6e
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -317,6 +317,13 @@ struct QCoreApplicationData {
     ~QCoreApplicationData() {
 #ifndef QT_NO_LIBRARY
         delete app_libpaths;
+#endif
+#ifndef QT_NO_QOBJECT
+        // cleanup the QAdoptedThread created for the main() thread
+        if (QCoreApplicationPrivate::theMainThread) {
+            QThreadData *data = QThreadData::get2(QCoreApplicationPrivate::theMainThread);
+            data->deref(); // deletes the data and the adopted thread
+        }
 #endif
     }