Purge Import::Offline::Configuration for completed exports/imports

When the export or import stage of an Offline Transfer is completed, the associated Import::Offline::Configuration record should be deleted.

This is to ensure that we don't store any credentials for longer than is strictly necessary.

This could either be done "on-demand", e.g. as a callback when the export moves into a certain state, or via a cron-job which purges the configurations of completed imports or exports on a regular basis.