### Problem
Big instances can have huge authorized_keys files when using OpenSSH instead of the internal ssh server. Forgejo always re-generates the contents of that file when a user is deleted, even if he does not even have a public key uploaded. In case of codeberg.org, a 15MB file gets rewritten. If we batch delete 100 Spam users without ssh keys, we rewrite 1.5GB, this takes time and wears the SSD. In addition, there is a high chance of hitting a race contidion bug, when deleting users in parallel.
### Solution / Mitigation
This patch prevents rewriting authorized_keys files, when not necessary. It greatly speeds up deleting malicious users, saves IO bandwidth and SSD wear. It also greatly reduces the chance of hitting a race condition bug. Fixing the race condition is not the scope of this patch though.
## Checklist
The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).
### Tests
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
- [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
- [ ] in `web_src/js/*.test.js` if it can be unit tested.
- [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).
### 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.
Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6097
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Andreas Shimokawa <shimokawa@fsfe.org>
Co-committed-by: Andreas Shimokawa <shimokawa@fsfe.org>
