Remove feature flag ci_child_pipeline_coverage_reports

What does this MR do and why?

Remove feature flag ci_child_pipeline_coverage_reports.

This feature flag has been enabled globally in GitLab.com. Removing the feature flag would release this feature in the next release.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363557 (closed)