[fix] delete language translations when deleting a language

This tries to implement https://github.com/joomla-projects/com_localise/pull/249 in a better way:

* Not using a helper to interact with DB. Table related stuff shoudl be inside table.
* Avoid to fully purge DB and then to regenerate files.

DB structure is not ok anyway. I think we should consider to add a language tag column to `#__localise` so everything would be easier.