x11: Remove text-scaling-factor setting checks

The text-scaling-factor setting is not useful, now that we watch the Xft/DPI and Gdk/WindowScalingFactor XSETTINGS keys; on the contrary, it is problematic in mixed environments with both the KDE and GNOME portals, as they end up stepping on each other's toes, and we end up with the wrong scaling factor value.

Fixes: https://github.com/libsdl-org/SDL/issues/11142

This MR removes a bunch of code, so additional testing is appreciated.

Copying my comment here, as I accidentally commented in the issue instead.

This works correctly for me in KDE+Xwayalnd and KDE+X11.

@ebassi Can you let know if this worked okay for you in both session types in gnome? If so, I think this should be good to merge.

Merged, thanks!