Allow obsolete config strings to be culled from localization table

Follow-up to https://github.com/backdrop/backdrop/pull/2397#discussion_r244554774

Because we have both $active_config and $staging_config, and you've passed in $context above... we could delete the out-of-date strings that are no longer used from the locale table when a string changes. For example if the translation for the registration email changes (or the content of the registration email changes), we don't need to be keeping all the old copies of that translated string in the database, just delete them all with that context key.