4962 when workbench import is deleted also delete associated referential if ready false
Created by: teddywing
When destroying a WorkbenchImport, if its associated NetexImports have an associated Referential with field ready: false, that Referential should be destroyed.
This is set up with a before_destroy callback directly on NetexImport.
We need to get rid of the non-ready Referentials because we can get into a situation where there's an error on the Referential, but since it has ready: false, it doesn't get displayed on the website, and thus the user has no clue what the problem is. Worse, these Referentials will cause subsequent re-imports to fail.