Cleanup after switching to dedicated signing key for CI_JOB_JWT
Initially when working on #214607 (closed) we took an approach to store the signing key in a file.
After some discussion (see #222690 (comment 398368920)) we decided to store it as application setting in the database. This was done with !43950 (merged).
Proposal
We have some clean up to do in order to remove code and keys we do not need anymore:
- Revert omnibus-gitlab!4158 (merged)
- Revert gitlab-org/charts/gitlab!1318 (merged)
- If possible remove the key generated by above MRs
- Remove key generated for GitLab.com with https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10573#note_363148693