gtk: nuke the legacy apprt from orbit

We don't really have any large outstanding regressions on -ng to warrant
keeping this alive anymore. ¡Adiós!

src/datastruct/split_tree.zig

@@ -1266,7 +1266,7 @@ pub fn SplitTree(comptime V: type) type {
 
         /// Make this a valid gobject if we're in a GTK environment.
         pub const getGObjectType = switch (build_config.app_runtime) {
-            .gtk, .@"gtk-ng" => @import("gobject").ext.defineBoxed(
+            .@"gtk-ng" => @import("gobject").ext.defineBoxed(
                 Self,
                 .{
                     // To get the type name we get the non-qualified type name