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

main_x11.cpp

@@ -408,6 +408,7 @@ KWIN_EXPORT int kdemain(int argc, char * argv[])
     setenv("QT_QPA_PLATFORM", "xcb", true);
 
     qunsetenv("QT_DEVICE_PIXEL_RATIO");
+    qunsetenv("QT_SCALE_FACTOR");
     QCoreApplication::setAttribute(Qt::AA_DisableHighDpiScaling);
 
     KWin::ApplicationX11 a(argc, argv);