Remove save() function from subscription blocks

[ice9js]

Fixes #26866.

Changes proposed in this Pull Request:

This patch fully removes the old save() implementation from the subscribe block, which has been rendered obsolete by #26947 where the block has been converted to be dynamic.

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?

Jetpack product discussion

N/A

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

No.

Testing instructions:

• Make sure Jetpack has a valid connection on your test site for subscribe blocks to be enabled.
• Create a new post and a subscribe block.
• Save.
• Preview the post, it should still feature the subscribe form you just added.
• Go back to the editor and switch the block to code editor.
• There should be no shortcode being generated for the block anymore.

[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 delete/subscribe-block-save to get started. More details: p9dueE-5Nn-p2

[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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Jetpack plugin:

• Next scheduled release: December 6, 2022.
• Scheduled code freeze: November 28, 2022.

[CGastrell]

Massive! Works for me!

[coder-karen]

Very nice!