[scsi] Make LUN a property of the SCSI backend only

Nothing within the SCSI core actually refers to the LUN, so we can
simplify matters by treating it as purely a property of the backend.

src/include/gpxe/scsi.h

 struct scsi_device {
 	/** Block device interface */
 	struct block_device blockdev;
-	/** Logical unit number (LUN) */
-	struct scsi_lun lun;
 	/**
 	 * Issue SCSI command
 	 *

src/net/tcp/iscsi.c

 	/* Attach parent interface, mortalise self, and return */
 	scsi->backend = ref_get ( &iscsi->refcnt );
 	scsi->command = iscsi_command;
-	scsi->lun = iscsi->lun;
 	ref_put ( &iscsi->refcnt );
 	return 0;