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/collab/src/tests/integration_tests.rs

@@ -4197,7 +4197,6 @@ async fn test_collaborating_with_lsp_progress_updates_and_diagnostics_ordering(
                 }],
             },
         );
-        executor.run_until_parked();
     }
     fake_language_server.notify::<lsp::notification::Progress>(&lsp::ProgressParams {
         token: lsp::NumberOrString::String("the-disk-based-token".to_string()),