vfio/pci: remove CONFIG_VFIO_PCI_ZDEV from Kconfig

In case we're running on s390 system always expose the capabilities for
configuration of zPCI devices. In case we're running on different
platform, continue as usual.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>