**Reason**:
The DeepSea skin was originally created a long time ago, which was more or less a Vector skin with only some slight appearance changes (there used to be some functionality changes too, but no more). So I re-implemented it as a pure-CSS theme for Vector pretty recently, in https://gerrit.wikimedia.org/r/#/c/359623/
- [x] Archive documentation on `mediawiki.org` - http://mediawiki.org/wiki/Skin:DeepSea
- [x] Remove from `mediawiki/skins` + `.gitmodules`
- [ ] Remove tests in `intregation/config` ( `zuul/layout.yaml` )
- [ ] Remove from `translatewiki` ( `groups/MediaWiki/mediawiki-skins.txt` )
- [x] Archive Phabricator project #DeepSea for associated extension/skin
- [x] Edit Phabricator project #DeepSea description for associated extension/skin with a link to this ticket
- [ ] Archive the extension's diffusion repository: https://phabricator.wikimedia.org/r/project/mediawiki/skins/DeepSea
- [x] Mark the repository read-only in Gerrit: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/skins/DeepSea