Windows: avoid using 'rem' in the nmake makefile

To avoid the possibility that someone creates rem.exe, rem.bat or
rem.cmd, simply don't use it.  In the cases it was used, it was to
avoid empty lines, but it turns out that nmake handles those fine, so
no harm done.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/6686)