Fix invalid swagger syntax of $ref with sibling (#7019)

Ticket #5676 states that the output from `make generate-swagger` is not valid according to the current [OpenAPI specification](https://swagger.io/specification/#path-item-object). In future this [may change](https://swagger.io/specification/#reference-object) but currently this has the effect that the `description` field will be ignored. Removing the description from the type definition of the offending entry will fix the invalid output and have the same end result as before, as the description would have been ignored anyway.

## Checklist
### Tests

No changes to Go (except removal of comment) or Javascript code. Have added the `lint-swagger` task to the CI test run however.

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7019
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Sven Steinbauer <sven@unlogic.co.uk>
Co-committed-by: Sven Steinbauer <sven@unlogic.co.uk>