pgsql: Remove configure probes for poll and poll.h.

Remove configure probes for poll and poll.h.

poll() and <poll.h> are in SUSv2 and all targeted Unix systems have
them.

Retain HAVE_POLL and HAVE_POLL_H macros for readability. There's an
error in latch.c that is now unreachable (since we always have one of
WIN32 or HAVE_POLL defined), but that falls out of a decision to keep
using defined(HAVE_POLL) instead of !defined(WIN32) to guard the poll()
code.

Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b79ec732d29fe42e91aeab4da62d446f226b594a

Modified Files
--------------
configure | 4 ++--
configure.ac | 2 --
src/include/pg_config.h.in | 6 ------
src/include/port.h | 2 ++
src/tools/msvc/Solution.pm | 2 --
5 files changed, 4 insertions(+), 12 deletions(-)