## Reason
#MediaWiki-extensions-LilyPond is vulnerable and obsolete; replaced by #mediawiki-extensions-score. The extension is already partially archived.
## To-Do list
- [x] Add maintainers of that extension as subscribers to this task
- [x] Archive documentation on `mediawiki.org` (https://www.mediawiki.org/wiki/Extension:LilyPond)
- [x] Archive Wikidata item associated with mediawiki.org documentation page (remove nearly all statements, except `instance of (P31) = MediaWiki extension (Q6805426)`, which should add a qualifier `end time (P582) = the YYYY-MM-DD date that you decided to archive extension, generally per edit history`, change English (and other languages if possible) description to "archived MediaWiki extension" (and its translation)) [[https://www.wikidata.org/w/index.php?title=Q21677233&oldid=721389234|diff 721389234 and else]]
- [x] Remove from `mediawiki/extensions`/`mediawiki/skins` (along with in `.gitmodules`). //Not there//
- [x] Remove tests in `integration/config` (`zuul/layout.yaml` ). //Cannot find it there//
- [ ] Remove from `translatewiki.net` (`groups/MediaWiki/mediawiki-extensions.txt`).
- [ ] Remove manifest/other references from `mediawiki/vagrant` if they exist.
- [x] Empty repository and add an ARCHIVED file. //No repo to archive//
- [x] Remove from https://doc.wikimedia.org/cover-extensions/ if present (requires #continuous-integration-infrastructure shell user to delete directly from the server) //Not listed//
- [x] Archive Phabricator project #MediaWiki-extensions-LilyPond for associated extension, and mark all tasks of that project either Declined or Invalid.
- [x] Edit Phabricator project #MediaWiki-extensions-LilyPond description for associated extension with a link to this ticket.
- [x] Archive the extension's diffusion repository: //None//
- [x] Mark the repository read-only in Gerrit: //None//
- [x] Delete the GitHub mirror: //None//