Winrt: QThread: Use CloseHandle instead of delete

Deletion of the handle might cause abort() to be
called while CloseHandle seems to be fine.

Change-Id: Id4903bb23cabc6c16f002c43f1db4ada36e32dda
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>