Doc: Clean up references to Qt System Info

The module is not released yet; the current documentation is misleading.

Change-Id: Ia20ac8e3504942f6f60b94264f3a12497e09da24
Reviewed-by: Niels Weber <niels.weber@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>