refactor: simplify SSR options' if statement

[Saeed-Paris]

Description

nothing fancy or important. just noticed that buildSsrCjsExternalHeuristics function had same statement in both if and else (Hope this wasn't a wierd edge case)

Happy first Vite contribution 🥳🥳🥳

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[patak-dev]

target is already 'node', we can remove this condition too. Note: We plan to remove CJS external heuristics support in Vite 5. If you are using it for some reason, please comment on #12466. We hope to move to ESM only for SSR as all services now properly support ESM.