Remove ci_jwt_signing_key application secret from the documentation

What does this MR do?

The documentation is out of sync. There were reverts and rework on this where initially there was Rails.application.secrets.ci_jwt_signing_key, then it was reverted, then finally added back as Gitlab::CurrentSettings.ci_jwt_signing_key.

This MR removes the line in the documentation that shows the ci_jwt_signing_key as an application secret.

See the original MR for more context.

Related issues

Author checklist

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Required

• Merge Request Title and Description are up to date, accurate, and descriptive
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added/updated
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
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