Blocking site header buttons's underline on hover

[ollips]

Task: task-1010346

Link: preview

Adding rules to disable underline in hover state.

Testing

1. Visit any page.
2. Make sure brand link on hover state doesn't have double underline
3. Review code

Contributor checklist

• Did you update the description of this pull request with a review link and test steps?
• Did you submit a changeset? Changesets are required for all code changes.
• Does your pull request implement complex UI logic (js/ts)? Consider adding an integration test to test your user flow.
• Does your pull request add a new page to the documentation site? Add your new page for automated accessibility testing in /integration/tests/accessibility.

[changeset-bot]

🦋 Changeset detected

Latest commit: 6093fff

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@microsoft/atlas-css	Minor

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