[libpng18] chore: Clean up remnants of the long-discontinued Win16 support

Remove #ifdef sections and other workarounds for old Windows compilers
that lacked proper support for Win32, including, especially, support
for the Win32 stdio API.

This is a cherry-pick of commit e936211760ddf0ed4a4711ea897b59395dfd206e
from branch 'libpng18'.

Reviewed-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>
6 files changed