virtio: remove unused id field from struct virtio_blk_outhdr

This field has been unused since an older version of virtio.  Remove
it now before we freeze the ABI.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au.

include/linux/virtio_blk.h

@@ -39,8 +39,6 @@ struct virtio_blk_outhdr
 	__u32 ioprio;
 	/* Sector (ie. 512 byte offset) */
 	__u64 sector;
-	/* Where to put reply. */
-	__u64 id;
 };
 
 #define VIRTIO_BLK_S_OK		0