Clean up empty account and container partitions directories.

Because we iterate over these directories on a replication run,
and they are not (previously) cleaned up, the time to start the
replication increases incrementally for each stale directory
lying around.  Thousands of directories across dozens of disks
on a single machine can make for non-trivial startup times.

Plus it just seems like good housekeeping.
Closes-Bug: #1396152

Change-Id: Iab607b03b7f011e87b799d1f9af7ab3b4ff30019