Remove the person.displayname column in the postgres backend

The person.displayname column was used to store the mailmaps override.

Now it's managed in the masking proxy, we should delete it.

marked this issue as related to swh/infra/sysadm-environment#5391 (closed)

it should be deleted indeed. Let me rename this issue accordingly then.

changed title from Relevance to keep the person.displayname column to Remove the person.displayname column in the postgres backend

changed the description