UnderlineNav2: Remove selectionVariant prop from stories and refactor accordingly

[broccolinisoup]

Reverting some changes back on UnderlineNav that was introduced #2878

We need to pass selectionVariant="single" on the ActionList in UnderlineNav because we call onSelect function. If we don't set it, it throws an error when an item is selected (ActionList ref)

To be able to hide the CheckIcon we can use styling. This is how it was originally done but I took this as an opportunity and update the styling to check a data component rather than relying on the node order in the dom.

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Merge checklist

• Added/updated tests
• Added/updated documentation
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	101.88 KB (+0.01% 🔺)
dist/browser.umd.js	102.4 KB (+0.04% 🔺)

[broccolinisoup]

Because the PR that introduced the problem that we are addressing here, is reverted I am closing this PR, and it is already been fixed in the new draft PR that re-introduces the changes.