vfio: Remove 'vfio-platform'

vfio: Remove 'vfio-platform'

The VFIO_PLATFORM device type has been deprecated in the QEMU 10.0
timeframe. All dependent devices have been removed. Now remove the
core vfio platform framework.

Rename VFIO_DEVICE_TYPE_PLATFORM enum to VFIO_DEVICE_TYPE_UNUSED to
maintain the same index for the CCW and AP VFIO device types.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250901064631.530723-8-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>