Remove the note about polyfill being a default behavior of the `@babel/plugin-transform-runtime`

[Andarist]

When I tried to transform with polyfill: false I got this runtime error: https://github.com/babel/babel/blob/aad7eb743d1354a5475f9d43226bfedc5f149e4b/packages/babel-plugin-transform-runtime/src/index.ts#L161-L165

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