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/storybook/src/storybook.rs

@@ -76,7 +76,7 @@ fn main() {
 
         let selector = story_selector;
 
-        let theme_registry = <dyn ThemeRegistry>::global(cx);
+        let theme_registry = ThemeRegistry::global(cx);
         let mut theme_settings = ThemeSettings::get_global(cx).clone();
         theme_settings.active_theme = theme_registry.get(&theme_name).unwrap();
         ThemeSettings::override_global(theme_settings, cx);