Summary:
In addition to removing the earlier partially implemented garbage collection
logic from the BlobDB codebase, the patch also removes the test cases (as well as
the related sync points, as appropriate) that were only relevant for the old
implementation, and reworks the remaining ones so they use the new GC logic.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6278

Test Plan: `make check`

Differential Revision: D19335226

Pulled By: ltamasi

fbshipit-source-id: 0cc1794bc9892feda1426ed5522a318f3cb1b692