Also unset QT_SCALE_FACTOR
Qt scaling and kwin on X11 is never going to work so it is disabled.
However, Qt uses multiple environment variables for subtly different
internal things. We don't set QT_SCALE_FACTOR in Plasma, using different ones.
An XFCE user was setting this, which kinda makes sense if your DE doesn't
have an appropriate UI.
I don't think it's relevant for main_wayland as wl_output.scale should
trump the environment variable.
Reviewed By: zzag
Subscribers: zzag, kwin
Differential Revision: https://phabricator.kde.org/D20234