Revert "Update default on XwaylandClientsScale to true"
This reverts commit c6265f23.
Scaling opt out is not production ready yet. There are still some issues in client-side decorated GTK apps.
This reverts enabling scaling opt out by default. The users can still enable it in the system settings if they want to test it.
In order to fix visual glitches in GTK apps, we would need to touch some sensitive code in rendering code path in kwin, which I don't think is worth rushing.