Only show "Force font DPI" feature on X11

Allowing users to manually change the font DPI independent of font sizes
and the global scale is already of questionable value on X11, but on
Wayland where Qt scaling is always used, it is an anachronism. So let's
only show it on X11. On Wayland, people can use the global scaling
feature in the KScreen KCM.

See also https://phabricator.kde.org/T9500