**Reason**:
The extension has a fatal error due to a change in the Contributors extension made two years ago 797468db000225acea8fcfc9fabb8c3b57b57128 (described at T195818).
https://wikiapiary.com/wiki/Extension:ContributorsAddon shows it is only used on three wikis, all of them are severly outdated.
- [x] Archive documentation on `mediawiki.org` https://www.mediawiki.org/wiki/Extension:ContributorsAddon
- [x] Remove from `mediawiki/extensions` ( along with in `.gitmodules`)
- [x] Remove tests in `integration/config` ( `zuul/layout.yaml` )
- [x] Remove from `translatewiki` ( `groups/MediaWiki/mediawiki-extensions.txt` )
- [ ] Remove manifest/other references from `mediawiki/vagrant` if they exist
- [ ] Empty repository and add a OBSOLETE file.
- [ ] ~~Archive Phabricator project <#ProjectX> for associated extension/skin~~
- [ ] ~~Edit Phabricator project <#ProjectX> description for associated extension/skin with a link to this ticket~~
- [ ] Archive the extension's diffusion repository: rECNA
- [ ] Mark the repository read-only in Gerrit: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/ContributorsAddon
- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-ContributorsAddon