Sonata Admin support should be removed since it's not working for at least 4 years

[Tetragramat]

In documentation https://github.com/Payum/Payum/blob/master/docs/symfony/configure-payment-in-backend.md#with-sonata-admin is specified support for sonata admin, but in code https://github.com/Payum/PayumBundle/blob/master/DependencyInjection/PayumExtension.php#L247 support for sonata admin is disabled by exception because it's broken for at least 4 years.

Issue about wrong documentation and questions how to make it work Payum/Payum#776