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

BUG: 442901


(cherry picked from commit ba1f944e)