Delete legacy code not used any more. (#8508)

Summary:
The removed function in this PR,  just only have declared and dose not have any reference used.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8508

Reviewed By: mrambacher

Differential Revision: D29649033

Pulled By: jay-zhuang

fbshipit-source-id: df98143b73d6c184a2a60c9f7ea2548a065ee35d

db/db_impl/db_impl.h

@@ -1189,11 +1189,6 @@ class DBImpl : public DB {
                               ColumnFamilyHandle* column_family,
                               const Slice* begin, const Slice* end);
 
-  Status GetApproximateSizesInternal(const SizeApproximationOptions& options,
-                                     ColumnFamilyHandle* column_family,
-                                     const Range* range, int n,
-                                     uint64_t* sizes);
-
   // The following two functions can only be called when:
   // 1. WriteThread::Writer::EnterUnbatched() is used.
   // 2. db_mutex is NOT held