DSD-1852: style cleanup for SubNav

[bigfishdesign13]

Fixes JIRA ticket DSD-1852

This PR does the following:

• Simplifies the styles for the SubNav component.
• Still need to address the dark mode styles.

How has this been tested?

• local Storybook

Accessibility concerns or updates

• n/a

Accessibility Checklist

• [ ] Checked Storybook's "Accessibility" tab for color contrast and other issues.
• [ ] The feature works with keyboard inputs including tabbing back and forward and pressing space, enter, arrow, and esc keys.
• [ ] For hidden text or when aria-live is used, a screenreader was used to verify the text is read.
• [ ] For features that involve UI updates and focusing on DOM refs, focus management was reviewed.
• [ ] The feature works when the page is zoomed in to 200% and 400%.

Open Questions

-

Checklist:

• [ ] I have updated the Storybook documentation and changelog accordingly.
• [ ] I have added relevant accessibility documentation for this pull request.
• [ ] All new and existing tests passed.

Front End Review:

• [ ] Review the Vercel preview deployment once it is ready.

[vercel[bot]]

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Name	Status	Preview	Comments	Updated (UTC)
nypl-design-system	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 6, 2024 8:44pm

[github-actions[bot]]

Your pull request is missing a changelog—was that intentional?