Reason: Per T174881#3770248
- Archive documentation on mediawiki.org [there was none]
- Remove from mediawiki/extensions (along with in .gitmodules)
- Remove tests in integration/config (zuul/layout.yaml)
- 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 the extension's diffusion repository: rEAES
- Mark the repository read-only in Gerrit.
- Delete the GitHub mirror. https://github.com/wikimedia/mediawiki-extensions-ActionEditSubmit