vfio: mdev: Remove unused function declarations

The definition of mdev_bus_register() and mdev_bus_unregister() have been
removed since commit 6c7f98b334a3 ("vfio/mdev: Remove vfio_mdev.c"). So,
let's remove the unused declarations.

Signed-off-by: Zhang Zekun <[e-mail]>
Reviewed-by: Jason Gunthorpe <[e-mail]>
Reviewed-by: Kevin Tian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alex Williamson <[e-mail]>

drivers/vfio/mdev/mdev_private.h

···
 #ifndef MDEV_PRIVATE_H
 #define MDEV_PRIVATE_H
 
-int  mdev_bus_register(void);
-void mdev_bus_unregister(void);
-
 extern const struct bus_type mdev_bus_type;
 extern const struct attribute_group *mdev_device_groups[];