Bug 1962978 - Use non-native listboxes on GTK. r=stransky,geckoview-reviewers,nika

Gtk4 doesn't use different background colors for GtkColumnView (the
replacement for the treeview), so avoid computing one.

Our computation didn't have any guarantee of matching the fieldtext
foreground color.

This matches windows too. Remove unused -moz-eventreerow while at it.

Differential Revision: https://phabricator.services.mozilla.com/D249237

servo/components/style/values/specified/color.rs

@@ -270,9 +270,8 @@ pub enum SystemColor {
     /// Used for menubar item text when hovered.
     MozMenubarhovertext,
 
-    /// On platforms where these colors are the same as -moz-field, use
-    /// -moz-fieldtext as foreground color
-    MozEventreerow,
+    /// On platforms where this color is the same as field, or transparent, use fieldtext as
+    /// foreground color.
     MozOddtreerow,
 
     /// Used for button text when pressed.