theme: Remove unused staff parameter for listing themes (#20077)

This PR removes the `staff` parameter for listing themes, as it was not
used.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -3424,7 +3424,7 @@ mod tests {
         theme::init(theme::LoadThemes::JustBase, cx);
 
         let mut has_default_theme = false;
-        for theme_name in themes.list(false).into_iter().map(|meta| meta.name) {
+        for theme_name in themes.list().into_iter().map(|meta| meta.name) {
             let theme = themes.get(&theme_name).unwrap();
             assert_eq!(theme.name, theme_name);
             if theme.name == ThemeSettings::get(None, cx).active_theme.name {