Remove gitlab-keys script

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25598, we started moving the responsibility of managing authorized_keys_file to gitlab-rails. But we have a fallback path which will still use the gitlab-keys script.

We are removing that fallback path in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32138. So here, we no longer need it and going to be removed as well.

The GitlabKeys class (now a module) still exists but the responsibility has been reduced. It's still being used by gitlab-shell-authorized-keys-check-ruby and gitlab-shell-authorized-principals-check-ruby. Once those ruby fallback scripts can be removed, we can completely remove GitlabKeys as well.

Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/60071