Remove the feature library modal's feedback link

What does this MR do and why?

Remove the feature library modal's feedback link

We have closed the feedback issue, so it does not make sense to keep linking to it from the modal. We'll likely create a new feedback issue at some point, so we'll eventually bring the link back. This removes the unnecessary code for now.

Changelog: removed

References

• Closed feedback issue: #604008 (closed)
• Tailored navigation: Concrete fixes and polish ... (#621643)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag: Feature.enable(:feature_library_modal)
2. Visit any project or group page
3. Open the Feature Library modal via the sidebar ("Feature catalog" button)

MR acceptance checklist

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