Remove dead handling for pre-POSIX sigwait().

Remove dead handling for pre-POSIX sigwait().

sigwait() is in SUSv2 and all targeted Unix systems have it.  An earlier
pre-standard function prototype existed on some older systems, but we
no longer need a workaround for that.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Greg Stark <stark@mit.edu>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com