Remove `performance_roadmap` feature flag and related legacy code from Roadmap app

What does this MR do and why?

This MR removes performance_roadmap, and older code from Roadmap app which was responsible for infinite horizontal scrolling of timeline, as we now have linear pagination instead. Also, this MR, once merged, will have a follow-up MR with removal of roadmap_daterange_filter feature flag.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Related to #337198 (closed)