General: remove backwards-compatibility WP 6.0 code

[jeherve]

Proposed changes:

Following #30120, let's remove code that was only in there to keep backwards-compatibility with WP 6.0.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

See #27795

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• Check impacted files, those features should keep working.
  • Go to Posts > Add New, and add a new Business Hours block
  • It should work with no js errors in the console, and regardless of your site language.

[github-actions]

Thank you for your PR!

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• ✅ Add testing instructions.
  
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Jetpack plugin:

• Next scheduled release: May 2, 2023.
• Scheduled code freeze: April 24, 2023.

[github-actions]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run

bin/jetpack-downloader test jetpack update/backwards-compat-61

to get started. More details: p9dueE-5Nn-p2