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>