Revert "Adwaita: old style toolbar buttons"
From the first commit message:
This reverts commit d827fe71.
The commit is not good enough because not all toolbar buttons are designed to be flat. In particular it breaks the linked button style inside toolbars. We need another way to flatten particular toolbar buttons.
Instead, this merge request uses has-frame
for particular toolbar buttons.
See also: !1728 (closed) & !2592 (closed)