Also unset QT_SCALE_FACTOR

Summary:
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.

BUG: 406195

Reviewers: zzag

Reviewed By: zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D20234