config: Remove mentions of deprecated `self_signed_cert` config

What does this MR do?

config: Remove mentions of deprecated self_signed_cert config

The self_signed_cert configuration has been deprecated by gitlab-shell in v14.8 and was fully removed in v15.0. Stop mentioning it.

Related issues

gitlab-org/gitlab-shell#120 (closed)

Checklist

See Definition of done.

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Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

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