Remove compat versionhelpers.h and remove cmake/configure check for it

Remove compat versionhelpers.h and remove cmake/configure check for it

The cmake file defined that file to be never present in contrast to the
old msvc-config.h that always had it present.
Remove also the compat implementation taken from mingw. All our current
build environments already have that header in place.

Change-Id: I9c85ccab6d51064ebff2c391740ba8c2d044ed1a
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20231128103950.62407-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27573.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a68595a582b2c6c220b4f4502753d5f4154000d8)