remove the force qt scaling env var

Now we use Qt scaling everywhere, to reduce the number of different code paths and visual differences between the X11 and Wayland sessions. In the past we didn't do this because there were too many bugs, but almost all of them have been fixed now.

The one thing this is known to regress is the mixed-DPI multimonitor use case. But that use case is already not supported on X11, so our story remains the same: if you have a mixed-DPI multimonitor setup, use Wayland.

BUG: 356446 FIXED-IN: 6.0