remove ENABLE_VC_BUILD checks, use v3 build output API for all vercel apps

[Rich-Harris]

As of today, all apps deployed to Vercel can use the Build Output API v3. It's no longer necessary to set the ENABLE_VC_BUILD environment variable, and instead of only opting into v3 behaviour when that environment flag is set, we can always use the v3 behaviour. This means we can delete all the v1 code.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

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[changeset-bot]

🦋 Changeset detected

Latest commit: 0ae59f0

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

This PR includes changesets to release 3 packages

Name	Type
@sveltejs/adapter-static	Patch
@sveltejs/adapter-vercel	Patch
@sveltejs/adapter-auto	Patch

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