chore: remove leftover Cloudflare Pages code and docs

[Nic-Polumeyv]

Stacked on #16604. Picks up the leftovers and adds a test for the new _routes.json check.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 1c1eb26:

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🦋 Changeset detected

Latest commit: 1c1eb26

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-cloudflare	Patch

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[Rich-Harris]

marking as draft since the branch it targets is also draft (to be merged post-v3)