Summary:
The code is dead in RocksDB as `log::Reader::initial_offset_` is always zero. We should delete it so we don't have to maintain it like in #4359.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4362

Differential Revision: D9817829

Pulled By: ajkr

fbshipit-source-id: 474a2c679e5bd273b40608f3a5332931d9eefe6d