Adwaita: no edge highlight

- Levelbars and progressbars are indicators, not controls. They don't
  need no affordance signifier. Make it flat!

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/2494

gtk/theme/Adwaita/_common.scss

@@ -3178,7 +3178,6 @@ levelbar {
   // level bar colours
   > trough > block {
     border: 1px solid;
-    box-shadow: inset 0 1px 0 0 rgba(255,255,255,0.2);
 
     &.low {
       border-color: if($variant == 'light', darken($warning_color, 10%), $borders_color);