Blocks: remove custom spinner styles

[jeherve]

Proposed changes:

This should ensure that the Spinner doesn't conflict with other spinners in the editor.

It was added in ed5175d, as part of #16302, but removing it doesn't seem to break the spinner's look.

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

References:

• Spinner: Enforce no background. WordPress/gutenberg#49695
• Update the Spinner design WordPress/gutenberg#37551
• p1683530650412459-slack-C0299DMPG

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

• No

Testing instructions:

• Check out this branch
• Go to Posts > Add New
• Add a Story block
• As you interact with the block, check that the loading spinner that appears is displayed properly.
• It should look good on the frontend as well

Here is an example post:
https://crucial-fennec.jurassic.ninja/2023/05/08/a-story/

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ✅ Add testing instructions.
  
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• ✅ Add changelog entries to affected projects
  

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Jetpack plugin:

• Next scheduled release: June 6, 2023.
• Scheduled code freeze: May 29, 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/spinner-styles

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