Trail Map: code clean-up

[southp]

Project thread: pau2Xa-5Ol-p2

Summary

This task is in general the execution phase of what we conclude from #89691 . It should consist of the following, but not restrict to:

• Implement the deploying plan concluded in Trail Map: Analyze the data and decide on the way forward #89691.
• Clean up the A/B test code introduced in Trail Map: Implement the A/B test. #89689.
• Clean up the feature flag introduced in Trail Map: Introduce a feature flag to gate the Trail Map version of plans grid. #89686.