Summary:
According to https://github.com/facebook/rocksdb/blob/6.27.fb/db/db_impl/db_impl.cc#L2896:L2911 and https://github.com/facebook/rocksdb/blob/6.27.fb/db/db_impl/db_impl_open.cc#L203:L208,
we are going to remove `iter_start_seqnum` and `preserve_deletes` starting from RocksDB 7.0

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9430

Test Plan: make check and CI

Reviewed By: ajkr

Differential Revision: D33753639

Pulled By: riversand963

fbshipit-source-id: c80aab8e8d8fc33e52472fed524ed703d0ffc8b6