@cem removed references to pdwait4(2) (a nonexistent syscall) in r339356.
This change cleans up additional references used strictly for the
nonexistent pdwait4(2) syscall.
From what I gather, the goal of this functionality was to simplify
monitoring/manipulating processes started with `pdfork`, et al. The syscall
was never completed though--just discussed on the capsicum mailing list back
in 2015:
https://lists.cam.ac.uk/pipermail/cl-capsicum-discuss/2015-May/msg00012.html .