Smarten up subEntities and return empty array for Entity items

[noplanman]

This PR cuts away a lot of bulky code and makes the sub entity generation smarter, i.e.

protected function subEntities()
{
    return [
        'data'        => [EncryptedPassportElement::class], // Will return an array of objects of this type.
        'credentials' => EncryptedCredentials::class,       // Will return a single object of this type
    ];
}

@php-telegram-bot/developers Have I missed anything by making this change? I think it's just so much more logical and easier to understand this way. And it makes the code more DRY, removing all the excessive bulk of identical methods.

[akalongman]

Would be great to test this manually after merging