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>