Remove custom root pool import code

Non-Linux OpenZFS implementations require additional support to be
used a root pool.  This code should simply be removed to avoid
confusion and improve readability.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #4951

include/sys/vdev_disk.h

@@ -37,11 +37,5 @@ typedef struct vdev_disk {
 	struct block_device	*vd_bdev;
 } vdev_disk_t;
 
-#ifndef __linux__
-extern int vdev_disk_physio(struct block_device *, caddr_t,
-			    size_t, uint64_t, int);
-extern int vdev_disk_read_rootlabel(char *, char *, nvlist_t **);
-#endif
-
 #endif /* _KERNEL */
 #endif /* _SYS_VDEV_DISK_H */