Remove references to keychain-mcd in Changes.rst

Remove references to keychain-mcd in Changes.rst

In commit 59e7e9fce8de6ea90d13baeaede83adc0b594e22 the contrib/keychain-mcd
code was completely removed.  But it was not removed from Changes.rst.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20171005212223.24694-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15561.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6255706295bf128ec5b5e4c1272fc6ffbfddf0ba)