Revert "lsp: Parse LSP messages on background thread - again (#23122)" (#23301)

This reverts commit 1b3b825c7f.

When debugging git diffs we found that this introduced a re-ordering of
messages sent to the LSP:

* User hits "format"
* Zed adjusts spacing, and sends "spaces changed" to the LSP
* Zed sends "format" to LSP

With the async approach here, the format request can now arrive before
the space changed request.

You can reproduce this with `test_strip_whitespace_and_format_via_lsp`
under some conditions.

Release Notes:

- N/A

crates/editor/src/test/editor_lsp_test_context.rs

@@ -315,12 +315,12 @@ impl EditorLspTestContext {
 
     pub fn handle_request<T, F, Fut>(
         &self,
-        handler: F,
+        mut handler: F,
     ) -> futures::channel::mpsc::UnboundedReceiver<()>
     where
         T: 'static + request::Request,
         T::Params: 'static + Send,
-        F: 'static + Send + Sync + Fn(lsp::Url, T::Params, gpui::AsyncAppContext) -> Fut,
+        F: 'static + Send + FnMut(lsp::Url, T::Params, gpui::AsyncAppContext) -> Fut,
         Fut: 'static + Send + Future<Output = Result<T::Result>>,
     {
         let url = self.buffer_lsp_url.clone();