chore: remove define merging

[Rich-Harris]

follow-up to #16812 — just curious to see what breaks if we remove this

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 2514975:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/2514975a1781e8f0f99060fbf6f4532884f2a884

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16819

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2514975

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16819-svelte.vercel.app/