Doc: Remove references to nonexistent functions

 - setAppDpiX()
 - setAppDpiY()
 - screen()

they only exist in Qt 4.8.

Change-Id: Ifca7cf9712a9b822b0776f413451d6bca7c69a37
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Martin Gräßlin <mgraesslin@kde.org>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>