cherry-pick-X11: Do not set the GDK_SCALE / GDK_DPI_SCALE environment variables

Merged Fushan Wen requested to merge cherry-pick-ba1f944e into Plasma/5.27

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)

BUG: 442901

(cherry picked from commit ba1f944e)

Merge request reports