Workflow revision for deleting an instrument name in a multi-source setting

Previously, we only considered one source and one contributor per instrument name, allowing defining _deletion of an instrument by the contributor_ (#274). With the definition in #436, instrument names can now be contributions from several people, each providing a different source or endorsing the same sources. Therefore, the deletion workflow should be reconsidered.

It is clear that a person should be able to remove their own contribution (here, the name-source relation they provided). Accordingly, an instrument name is deleted only when all contributors remove their contributions.

However, what about other aspects and the interplay of superuser?

Review the prior deletion definition in #274 and the multi-contributor definition in #436 before deciding how the workflow should behave. The work is done when the project has an agreed deletion policy covering removal of an individual's name-source contribution, deletion after all contributions are removed, and the role of a superuser.

Written by the indexing model from the issue text.