chore: remove Rspack config schema env variable

[chenjiahan]

Summary

Remove the RSPACK_CONFIG_VALIDATE variable, it is no longer used since Rspack v1.6.0-beta.1.

Related Links

• perf: remove JS side config validation rspack#11897

Checklist

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

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[Copilot]

Pull Request Overview

This PR removes the RSPACK_CONFIG_VALIDATE environment variable handling, which is no longer needed as of Rspack v1.6.0-beta.1. The change cleans up code and documentation that previously managed Rspack's config schema validation.

• Removed code that sets the environment variable to 'loose-silent' by default
• Removed documentation sections explaining the config validation behavior
• Removed explicit 'strict' validation enablement in e2e tests

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/core/src/plugins/basic.ts	Removed code that set RSPACK_CONFIG_VALIDATE to 'loose-silent'
website/docs/zh/guide/migration/rsbuild-0-x.mdx	Removed Chinese documentation section about Rspack config validation
website/docs/en/guide/migration/rsbuild-0-x.mdx	Removed English documentation section about Rspack config validation
e2e/playwright.config.ts	Removed RSPACK_CONFIG_VALIDATE environment variable assignment

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