remove the --disable-multi config switch

remove the --disable-multi config switch

This switch is broken and unmaintained.
However there wasn't any ticket about it so far,
which means that it is practically unused.

Get rid of it and simplify P2MP logic.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170816132454.13046-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15275.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 299a8f8f1aa10b5b0d006ae77c26de33d55d4a25)