Fix spinner in activity indicator (#54977)

Quick fix for a little regression I introduced in
https://github.com/zed-industries/zed/pull/54791 accidentally removing
the rotating spinner icon in the activity indicator.

Release Notes:

- N/A

crates/activity_indicator/src/activity_indicator.rs

@@ -729,13 +729,7 @@ impl Render for ActivityIndicator {
                         }
                     })
                     .label_size(LabelSize::Small)
-                    .when(content.icon.is_some(), |this| {
-                        this.start_icon(
-                            Icon::new(IconName::LoadCircle)
-                                .color(Color::Muted)
-                                .size(IconSize::Small),
-                        )
-                    })
+                    .loading(content.icon.is_some())
                     .map(|button| {
                         if truncate_content {
                             button.tooltip(Tooltip::text(content.message))