cherry-pick-X11: Do not set the GDK_SCALE / GDK_DPI_SCALE environment variables
The primary method for setting the global and text scale for GTK
applications is by using the gdk-window-scaling-factor and
gdk-unscaled-dpi settings. Those are also read by GTK4, which
at the same time dropped usage of GDK_DPI_SCALE.
Note that the scaling settings are only relevant on X11; On Wayland the DPI scale is communicated directly by KWin to applications on a per-monitor basis.
See also kde-gtk-config!49 (merged)
(cherry picked from commit ba1f944e)