Revert "Add undefined to JSON.stringify return type (#51897)"

[sandersn]

This reverts commit c7f49bc because of breaks like these found in github repositories:

function x(obj: object, key: string) {
    encodeURIComponent(JSON.stringify(obj[key as keyof typeof obj]))
}
function y(value: string) {
    if (typeof value !== 'string') {
        value = JSON.stringify(value)
    }
}
function z(file: PathOrFileDescriptor, obj: any, options: WriteFileOptions) {
    writeFileSync(file, JSON.stringify(obj, options?.replacer, options?.spaces), options)
}

(The last example depends on @types/fs-extra)

Discussed at the January 18 design meeting.

[ecyrbe]

Will another fix for undefined be attempted ?