Remove 'ci_jwt_signing_key' secret migrated to ApplicationSetting

What does this MR do?

The secret was actually never used:

1. Introduced in gitlab-org/gitlab!34249 (diffs) and !1318 (diffs)
2. The first MR was reverted by gitlab-org/gitlab!34646 (diffs) but the second was never reverted
3. The final solution was to add an ApplicationSetting: gitlab-org/gitlab!43950 (merged)

The secret documentation is also removed from gitlab at gitlab-org/gitlab!165829 (merged), and omnibus-gitlab at gitlab-org/omnibus-gitlab!7930 (merged).

Related issues

gitlab-org/gitlab#451151 (closed)

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