fix(theme-common): remove useless useSyncExternalStore shim

[slorber]

Motivation

Follow-up of #8961

With React 18, the useSyncExternalStore shim is not necessary anymore.

I forgot to remove it in the React 18 PR, which can lead to some conflict errors during SSR on sites upgrading to v3.

Test Plan

CI

Test links

Deploy preview: https://deploy-preview-9276--docusaurus-2.netlify.app/

[netlify]

✅ [V2]

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🔨 Latest commit	a4789b2
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 58	🟢 97	🟢 92	🟢 100	🟠 89	Report
/docs/installation	🟠 69	🟢 98	🟢 92	🟢 100	🟠 89	Report

[github-actions]

Size Change: -825 B (0%)

Total Size: 1.11 MB

Filename	Size	Change
website/build/assets/js/main.********.js	836 kB	-825 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	117 kB
website/build/assets/css/styles.********.css	113 kB
website/build/index.html	41.4 kB

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No change detected	-	Sep 4, 2023, 2:13 PM