Investigate `cached_route_lookups` feature flag

Problem

The feature flag is disabled on GitLab.com but enabled on staging.

It was introduced in Cache path lookups for namespaces [RUN ALL RSPE... (!57027 - merged).

Proposal

Review the code behind the feature flag and make a decision what to do next.

We can proceed with the rollout if the code still makes sense. Otherwise we should remove the feature flag.