Remove obsolete function declaration (#8724)

Summary:
Function `Version::UpdateFilesByCompactionPri()` is never called and not implemented.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8724

Reviewed By: ajkr

Differential Revision: D30643943

Pulled By: riversand963

fbshipit-source-id: 174b2d9a2a42e286222909a035cc74a7b5602335

db/version_set.h

@@ -851,11 +851,6 @@ class Version {
   // This accumulated stats will be used in compaction.
   void UpdateAccumulatedStats(bool update_stats);
 
-  // Sort all files for this version based on their file size and
-  // record results in files_by_compaction_pri_. The largest files are listed
-  // first.
-  void UpdateFilesByCompactionPri();
-
   ColumnFamilyData* cfd_;  // ColumnFamilyData to which this Version belongs
   Logger* info_log_;
   Statistics* db_statistics_;