Remove ignore rule for GpgSignature#valid_signature

What does this MR do?

Removes the ignore rule of the dropped column (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13771), as per https://docs.gitlab.com/ee/development/what_requires_downtime.html#step-2-removing-the-ignore-rule.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

https://docs.gitlab.com/ee/development/what_requires_downtime.html#step-2-removing-the-ignore-rule.

The development of this MR is sponsored by @siemens (/cc @bufferoverflow).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

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