We really only need to have one way to cleanup empty suffix dirs, and
that's normally during suffix hashing which only happens when invalid
suffixes get rehashed.
When we iterate a suffix tree using yield hashes, we may discover an
expired or otherwise reapable hashdir - when this happens we will now
simply invalidate the suffix so that the next rehash can clean it up.
This simplification removes an mis-behavior in the handling between the
normal suffix rehashing cleanup and what was implemented in ssync.
Change-Id: I5629de9f2e9b2331ed3f455d253efc69d030df72
Related-Change-Id: I2849a757519a30684646f3a6f4467c21e9281707
Closes-Bug: 1816501
585bf40cc0