Revert "theme: Turn ThemeRegistry into a trait (#20076)" (#20094)

This PR reverts #20076 to turn the `ThemeRegistry` back into a regular
struct again.

It doesn't actually help us by having it behind a trait.

Release Notes:

- N/A

crates/settings_ui/src/appearance_settings_controls.rs

@@ -83,7 +83,7 @@ impl RenderOnce for ThemeControl {
             "theme",
             value.clone(),
             ContextMenu::build(cx, |mut menu, cx| {
-                let theme_registry = <dyn ThemeRegistry>::global(cx);
+                let theme_registry = ThemeRegistry::global(cx);
 
                 for theme in theme_registry.list_names() {
                     menu = menu.custom_entry(