Feature to remove unused EFO terms from depo-curation

[eks-ebi]

As a curator, I want to be able to remove an EFO term from the list of terms in depo-curation when that term is no longer supposed to be used for annotation (e.g. if the term has been made obsolete in the source ontology, or if curators have identified a more appropriate term for that trait). At the moment, we can delete terms from the list, but only if they have never been assigned to a study. This is because, even if a trait has been removed from the current version of a study, it still remains assigned as a property of a previous version of that study, in the version history - and this prevents the curator from deleting the term.

There are different ways that this could be resolved: e.g. use a "soft" delete approach that removes the unused EFO from the list of available EFOs, without actually deleting it from the database

This will need more discussion before choosing an approach.