[heap] Remove unprotected_memory_chunks_mutex_

This mutex wasn't really used anymore. This should also speed up
code object allocation a bit.

Bug: v8:11888
Change-Id: I8ddc2ecc1aec74e8eb3e2d4b96354c50f3bff350
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2966382Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75185}

src/heap/heap.cc

@@ -2613,7 +2613,6 @@ void Heap::ComputeFastPromotionMode() {
 
 void Heap::UnprotectAndRegisterMemoryChunk(MemoryChunk* chunk) {
   if (unprotected_memory_chunks_registry_enabled_) {
-    base::MutexGuard guard(&unprotected_memory_chunks_mutex_);
     if (unprotected_memory_chunks_.insert(chunk).second) {
       chunk->SetReadAndWritable();
     }

src/heap/heap.h

@@ -2427,7 +2427,6 @@ class Heap {
 
   HeapObject pending_layout_change_object_;
 
-  base::Mutex unprotected_memory_chunks_mutex_;
   std::unordered_set<MemoryChunk*> unprotected_memory_chunks_;
   bool unprotected_memory_chunks_registry_enabled_ = false;