Rollup merge of rust-lang#136858 - safinaskar:parallel-cleanup-2025-02-11-07-54, r=SparrowLii

Parallel-compiler-related cleanup

Parallel-compiler-related cleanup

I carefully split changes into commits. Commit messages are self-explanatory. Squashing is not recommended.

cc "Parallel Rustc Front-end" rust-lang#113349

r? SparrowLii

``@rustbot`` label: +WG-compiler-parallel

Author: jhpratt

src/back/lto.rs

@@ -710,10 +710,6 @@ pub struct ThinBuffer {
     context: Arc<SyncContext>,
 }
 
-// TODO: check if this makes sense to make ThinBuffer Send and Sync.
-unsafe impl Send for ThinBuffer {}
-unsafe impl Sync for ThinBuffer {}
-
 impl ThinBuffer {
     pub(crate) fn new(context: &Arc<SyncContext>) -> Self {
         Self { context: Arc::clone(context) }