vdpa: remove unnecessary 'default n' in Kconfig entries

'default n' is not necessary since it is already the default when
nothing is specified.

Suggested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-2-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>