Remove redundant code for getnameinfo() replacement

Our getnameinfo() replacement implementation in getaddrinfo.c failed
unless NI_NUMERICHOST and NI_NUMERICSERV were given as flags, because
it doesn't resolve host names, only numeric IPs.  But per standard,
when those flags are not given, an implementation can still degrade to
not returning host names, so this restriction is unnecessary.  When we
remove it, we can eliminate some code in postmaster.c that apparently
tried to work around that.