Summary:
This interface is redundant and has been deprecated for a while.
It's also unused internally. Let's delete it.

I moved the comments to the corresponding functions in BackupEngine/
BackupEngineReadOnly. This caused the diff tool to not work cleanly.

Test Plan:
unit tests

  $ ./backupable_db_test

Reviewers: yhchiang, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D56331