chore: remove code related to splitVendorChunkPlugin

[sapphi-red]

Description

Follow up to #589

fixes this ecosystem-ci failure: https://github.com/vitejs/vite-ecosystem-ci/actions/runs/16640378447/job/47089473692

manualChunks breaks the execution order and the way rollup handles it and rolldown handles it is different. See rolldown/rolldown#5536

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[pkg-pr-new]

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commit: 3c4ca23