Writing prompts: remove placeholder prompt and fetch prompts by id

[creativecoder]

Proposed changes:

• Removes the placeholder writing prompt and related setting
• Updates the editor extension when the answer_prompt query param is used to fetch the prompt by id, so that we can support older links from notifications or emails.

Part of Automattic/wp-calypso#71938

Requires D97676-code

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Jetpack product discussion

pe7F0s-uo-p2

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[github-actions]

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[github-actions]

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[creativecoder]

Closing this... it looks like we'll be going a different direction with the editor extension (potentially with a dedicated writing prompt block), so I'll break out the setting/placeholder removal changes from here into a new, smaller PR.

[creativecoder]

I've spun out PR #28387 with the placeholder prompt removal and tag addition.