Adwaita: no edge highlight

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

Fixes #2494
9 jobs for wip/jimmac/levelbar-punch in 45 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  .Pre
passed #671911
style-check-diff

00:01:56

 
  Build
passed #671912
fedora-x86_64

00:07:17

passed #671914
win32
msys2-mingw32

00:24:59

passed #671913
release-build

00:04:53

 
  Analysis
passed #671915
static-scan

00:12:55

 
  Docs
passed #671916
reference

00:05:52

 
  Flatpak
manual #671917
flatpak allowed to fail manual
flatpak-manual:demo
manual #671919
flatpak allowed to fail manual
flatpak-manual:icon-browser
manual #671918
flatpak allowed to fail manual
flatpak-manual:widget-factory