docs(plugin-legacy): remove regenerator-runtime note

[sapphi-red]

Description

This note was added by #3389 for #3362.

But regenerator-runtime was included without using additionalLegacyPolyfills with vite@2.3.0 + plugin-legacy@1.3.4.
https://stackblitz.com/edit/vitejs-vite-wedvpk?file=package.json&terminal=dev
So I feel #3389 was something like #7819 (comment). (There was no reproduction so I cannot confirmed though)

Also regenerator-runtime is included without using additionalLegacyPolyfills with the latest version as reported by #7987.
I think this note should be removed.

fixes #7987

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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