Remove ci_limit_complete_hierarchy_size feature flag

What does this MR do and why?

This MR removes the ci_limit_complete_hierarchy_size feature flag.

• First added and enabled for GitLab SaaSin %15.4 !95857 (merged)
• Enabled by default in %15.5 !97841 (merged)
• Had it's configuration process iterated on in %15.7 !105119 (merged)
• And hasn't produced any unexpected behavior or operational problems so far in %15.8

So I'm comfortable removing it.

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.