Revert "sum_tree: Replace rayon with futures (#41586) (#41846)

This causes the background executor to hang

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/workspace/src/workspace.rs

@@ -7580,13 +7580,13 @@ pub fn create_and_open_local_file(
     path: &'static Path,
     window: &mut Window,
     cx: &mut Context<Workspace>,
-    default_content: impl 'static + Send + FnOnce(&mut AsyncApp) -> Rope,
+    default_content: impl 'static + Send + FnOnce() -> Rope,
 ) -> Task<Result<Box<dyn ItemHandle>>> {
     cx.spawn_in(window, async move |workspace, cx| {
         let fs = workspace.read_with(cx, |workspace, _| workspace.app_state().fs.clone())?;
         if !fs.is_file(path).await {
             fs.create_file(path, Default::default()).await?;
-            fs.save(path, &default_content(cx), Default::default())
+            fs.save(path, &default_content(), Default::default())
                 .await?;
         }