pgsql: Remove wal_sync_method=fsync_writethrough on Windows.

Remove wal_sync_method=fsync_writethrough on Windows.

The "fsync" level already flushes drive write caches on Windows (as does
"fdatasync"), so it only confuses matters to have an apparently higher
level that isn't actually different at all.

That leaves "fsync_writethrough" only for macOS, where it actually does
something different.

Reviewed-by: Magnus Hagander <magnus(at)hagander(dot)net>
Discussion: https://postgr.es/m/CA%2BhUKGJ2CG2SouPv2mca2WCTOJxYumvBARRcKPraFMB6GSEMcA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d0c28601efaa8b04f11a19506f3371bba8438acc

Modified Files
--------------
doc/src/sgml/wal.sgml | 5 ++---
src/backend/storage/file/fd.c | 6 ++----
src/bin/pg_test_fsync/pg_test_fsync.c | 4 +---
src/include/port/win32_port.h | 8 --------
4 files changed, 5 insertions(+), 18 deletions(-)