Remove the deprecated -balloon option

Remove the deprecated -balloon option

The "-balloon" option has been replaced by "-device virtio-balloon".
It's been marked as deprecated since two releases, and nobody
complained, so let's remove it now.

Acked-by: Paolo Bonzini <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Acked-by: Peter Krempa <[email protected]>
Acked-by: Ján Tomko <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>