test: remove unnecessary NODE_ENV override

[hi-ogawa]

Description

I noticed that currently pnpm -C playground/optimize-deps build is failing potentially due to this NODE_ENV override. The playground tests seem to be passing locally without it, so I'm wondering if this trick might be obsolete now. (ref: #8218 (comment), #9058, #9066)

Adding console.log("[NODE_ENV]", process.env.NODE_ENV) in vite.config.js, this is how it looks:

$ pnpm test-build playground/optimize-deps 
...

stdout | file:/home/hiroshi/code/others/vite/playground-temp/optimize-deps/playground-temp/optimize-deps/vite.config.js:8:9
[NODE_ENV] production


$ pnpm test-serve playground/optimize-deps 
...

stdout | file:/home/hiroshi/code/others/vite/playground-temp/optimize-deps/playground-temp/optimize-deps/vite.config.js:8:9
[NODE_ENV] development

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• 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).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

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[patak-dev]

Sounds good, thanks for checking this out @hi-ogawa!