Decommission Loki

#2761 (closed) has some details on who may still be using Loki but at a quick skim, it doesn't look like anyone.

https://gitlab.com/gitlab-com/gl-infra/observability/team/-/issues/4018 has determined that Loki is not the way forward for our logging stack, so we should do two things.

1. Determine the present cost of Loki.
2. Shut down everything we have.