refactor(theme-classic): remove span wrappers from layout links

[lex111]

Motivation

Applying changes from Infima - facebookincubator/infima#214

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Preview (/tests pages)

Related PRs

#6846

[netlify]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	69
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-6985--docusaurus-2.netlify.app/

[github-actions]

Size Change: -565 B (0%)

Total Size: 805 kB

Filename	Size	Change
website/build/assets/js/main.********.js	612 kB	-58 B (0%)
website/build/index.html	38.3 kB	-507 B (-1%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	49.9 kB
website/build/assets/css/styles.********.css	105 kB

_{compressed-size-action}

[slorber]

thanks 👍