write.2: sweep error code referencing disklabel(8)

The "-W" option for disklabel(8) was retired in:

f080d33b66306419b40c2f2a879198d3d5a8d36c

The ioctl used by the "-W" option, DIOCWLABEL, was retired in:

5777c5b9895805c33f1da4f72a3b5b297727888b