security/openvpn: drop man source patch

There has been a report of sporadic man-page rebuilds on OpenZFS.
While the patch order is correct, we do not intend to rebuild the
manpage (after a nobody -> openvpn change, for instance), and
we also patch the output files.  So just remove the source patch.

This should go without any functional changes, so ships without
bumping PORTREVISION.

There is an upstream ticket reporting a missing source file
in the tarball. https://community.openvpn.net/openvpn/ticket/1461

Reported by:    Jan Martin Mikkelsen
PR:             263116