Get rid of README.IPv6 and TODO.IPv6

Get rid of README.IPv6 and TODO.IPv6

README.IPv6 is quite useless because IPv6 is not a second
class citizen anymore. Most of the content is "obvious" or explained in
the manpage along with other details/options.

TODO.IPv6 is old and many implemented things are still reported there
for no clear reason. Delete file and report still open items in our
tracking system.

Cc: Gert Doering <gert@greenie.muc.de>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220207155757.22314-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23729.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>