Summary:
Periodic syncing of blob files is performed by `WritableFileWriter`;
`bytes_per_sync_` and `next_sync_offset_` in `BlobLogWriter` are
actually unused (or more precisely, only used by methods that are
themselves unused). The patch removes all this dead code.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7125
Test Plan: `make check`
Reviewed By: riversand963
Differential Revision: D22531021
Pulled By: ltamasi
fbshipit-source-id: 6b293ad5a79d3e6bf15c5c68f7aedd7ce7a15f10
bdf4de6cb9