1) When a backend is removed, the db instance directory was removed
as well (See also 463774 - index files for database should be deleted
when db is deleted). In case DB_RECOVER_FATAL is set in the DB open
after the removal (e.g., in restore), the logs in the transaction
logs are replayed and compared with the contents of the DB files.
At that time, if the db instance directory does not exist, libdb
returns FATAL error. To prevent the problem, we have to leave the
2) When removing index files, we don't have to open index files
with CREAT flag.