Cleanup use_gitaly_pagination_for_refs feature flag

Gavin Hinfeyrequested to merge
375737_cleanup_use_gitaly_pagination_for_refs_ff into master

What does this MR do and why?

Removes all references to the use_gitaly_pagination_for_refs feature flag.

Issue: #375737 (closed)

MR acceptance checklist

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

Edited by Gavin Hinfey

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