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 gitlab-org/charts/gitlab!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/support is also removed from gitlab at gitlab-org/gitlab!165829 (merged), charts/gitlab at gitlab-org/charts/gitlab!3910 (merged), and omnibus-gitlab at gitlab-org/omnibus-gitlab!7930 (merged).

Author's Checklist

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Required

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Expected

• Test plan indicating conditions for success has been posted and passes.
• Documentation is created or updated.
• Tests are added.

Related issues

gitlab-org/gitlab#451151 (closed)