drop support for DECnet

drop support for DECnet

DECnet belongs in the history museum of dead protocols along
with Appletalk and IPX.

Linux support has outlived its natural life and the time has
come to remove it from iproute2. Dead code is a source
of bugs and exploits.

If anyone actually has DECnet running on some old distribution
they can just keep to the old version of iproute2.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David Ahern <dsahern@gmail.com>