Remove the feature flag use_sub_repositories_api
What does this MR do and why?
This MR removes the feature flag use_sub_repositories_api
.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #392372 (closed)