Adwaita: no intention to make circular buttons universal

- No intention to make any button circular with a magic class
  Maybe if Lapo returns, he'll make all your dreams come true.

"Fixes" https://gitlab.gnome.org/GNOME/gtk/issues/861

gtk/theme/Adwaita/_common.scss

@@ -870,7 +870,7 @@ button {
     // bad with a 100% border radius (see https://bugzilla.gnome.org/show_bug.cgi?id=771205 for details),
     // so on relevant states we an additional background-image with a gradient clipped on the border-box,
     // so setting a transparent border would reveal it.
-    // FIXME: take care of colored circular button.
+
     $_border_bg: linear-gradient(to top, $alt-borders-color 25%, $borders-color 50%);
 
     &:not(.flat):not(:checked):not(:active):not(:disabled):not(:backdrop) {