Remove the broken qToVoidFuture() function

QFuture already has a constructor from QFuture<T> to QFuture<void>. It's
safe to remove this function, since it couldn't be ever used: it tries
to access QFuture's private member, which leads to compilation error. It
was never documented or tested.

Change-Id: Iaed9602e2737455d159012ea18ca241954e1f7df
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/thread/qfuture.h

@@ -431,12 +431,6 @@ inline QFuture<void> QFutureInterface<void>::future()
     return QFuture<void>(this);
 }
 
-template <typename T>
-QFuture<void> qToVoidFuture(const QFuture<T> &future)
-{
-    return QFuture<void>(future.d);
-}
-
 Q_DECLARE_SEQUENTIAL_ITERATOR(Future)
 
 QT_END_NAMESPACE