Remove stats_ field from SstFileManagerImpl (#13757)

Summary:
`SstFileManager` is supposed to be thread-safe for all of its public methods, but `SetStatisticsPtr` leads to a race condition because the access to `stat_` is not synchronized. We don't use `stat_` internally so we can get rid of it.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/13757

Test Plan: Existing unit tests.

Reviewed By: mszeszko-meta

Differential Revision: D77962592

Pulled By: archang19

fbshipit-source-id: e8e56194dda034935ddef44e479243770a73d065

file/sst_file_manager_impl.h

@@ -162,7 +162,6 @@ class SstFileManagerImpl : public SstFileManager {
   void Close();
 
   void SetStatisticsPtr(const std::shared_ptr<Statistics>& stats) override {
-    stats_ = stats;
     delete_scheduler_.SetStatisticsPtr(stats);
   }
 
@@ -216,7 +215,6 @@ class SstFileManagerImpl : public SstFileManager {
   std::list<ErrorHandler*> error_handler_list_;
   // Pointer to ErrorHandler instance that is currently processing recovery
   ErrorHandler* cur_instance_;
-  std::shared_ptr<Statistics> stats_;
 };
 
 }  // namespace ROCKSDB_NAMESPACE