After fiddleing a little too much with one of our subwikis (especially renaming), we seemingly have a zombie database. All our subwikis are functioning as expected (at least as far as we can see, even the renamed one). Still, when checking for upgrades for all our subwiki extensions from the main wiki admin section, we get a bunch of messages like these:
Starting job of type [upgradeplan] with identifier [extension/plan/] Resolving extension [Office Importer Application 13.10.5] on namespace [<OUTDATED_SUBWIKI_NAME>] Since database [<OUTDATED_SUBWIKI_NAME>] needs to be migrated, it couldn't be safely used! Please check your configuration to enable required migration for upgrading database from version  to version  [...]
where <OUTDATED_SUBWIKI_NAME> is an intermediate name of the aforementioned subwiki.
Obviously we did an upgrade recently, which after some hard work, finished as expected and runs smoothly now for the main and all other subwikis. Before the upgrade we occasionally had error messages concerning this zombie subwiki as well.
What would be the best way to get rid of this database? What could be the reason for such a left over? Are there any potential dangers involved in deleting the database in mysql or better, where should we erase it, to get rid of the messages without causing trouble elsewhere?
Any suggestions and hints much appreciated!