Summary:
Periodic syncing of blob files is handled by a lower layer, namely by
`WritableFileWriter`; the `NeedsFsync` method of `BlobFile` and the
`last_fsync_` member variable are actually unused and thus can be
removed. See also https://github.com/facebook/rocksdb/pull/7125 .
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7138
Test Plan: `make check`
Reviewed By: zhichao-cao
Differential Revision: D22562981
Pulled By: ltamasi
fbshipit-source-id: c235aad94a7c27120528c9ec270a7a5b9154e49f