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>
test: Remove dead code around QX11Info::setAppDpi{X,Y}

These functions don't exist for QX11Info.

Change-Id: Icb9a4b28c1f97ac8d59c93a29633547c84f6589c
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: David Faure <david.faure@kdab.com>