A bulkdata dir may get created for a new stream with an empty or
corrupted _format, before any data gets actually written. In that
case, we can just delete the new stream; worst case, we lose some
Note: The info in _format should really get moved into the database.
This was born when bulkdata switched from PyTables to a custom storage
system, and was probably stored this way to avoid tying the main DB
to specific implementation details while they were still in flux.