Cleaned up experiment `cross_stage_fdm`

What does this MR do and why?

Implements #351851 (closed)

The goal is the clean up the experiment cross_stage_fdm by removing it completely.

Screenshots or screen recordings

Before	After

How to set up and validate locally

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

1. Enable feature flag Feature.enable(:cross_stage_fdm)
2. Navigate to a group
3. Click the help dropdown in header bar
4. See feature discovery option no longer present
5. Feature discovery page has been removed and URL http://127.0.0.1:3000/groups/<group>/-/discover_premium_and_ultimate returns 404

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 #351851 (closed)