fix!(plugin-lightningcss): remove minify option

[chenjiahan]

Summary

Remove the minify option of @rsbuild/plugin-lightningcss. Rsbuild core now uses LightningCSS as the default minimizer, so we do not need to provide minify option anymore.

Related Links

#2716

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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