vfio: mdev: Remove unused function declarations

The definition of mdev_bus_register() and mdev_bus_unregister() have been
removed since commit 6c7f98b3 ("vfio/mdev: Remove vfio_mdev.c"). So,
let's remove the unused declarations.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240812120823.10968-1-zhangzekun11@huawei.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>