Errors deleting a corpus with datasets

Refs !2134 (comment 207336)

Deleting some corpora will cause IntegrityError because of above relations (table to delete, references):

• "documents_element"
  • training_datasetelement.element_id
  • process_process.element_id
• "documents_elementtype"
  • process_process.folder_type_id
• "ponos_task"
  • ponos_task.parents

i got those errors with that corpus : tests-20230928-113559.sqlite