Remove the feature flag ci_docker_image_pull_policy

What does this MR do and why?

This MR removes the feature flag ci_docker_image_pull_policy.

• Feature issue: #21619 (closed)
• Feature flag issue: #363186 (closed)
• Introduced in: !85588 (merged)
• Enabled by default in v15.2: !91403 (merged)

MR acceptance checklist

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