xsettings: remove check for native resolution when determining hi-dpi

A non-native resolution, can still be hi-dpi - e.g., on my laptop,
gnome-settings-daemon lists a resolution of 1920x1440 on my 2560x1440
display. The other checks we have in place should be sufficient to
disable hi-dpi support if the user picks a small resolution.

https://bugzilla.gnome.org/show_bug.cgi?id=709859