Remove Decnet Support from Kernel

HECNet still uses DECNet. See: http://mim.stupi.net/hecnet.htm

Unfortunate timing now that OpenVMS for x86 is available: https://vmssoftware.com/about/news/2022-07-14-openvms-v92-fo...

(Of course, OpenVMS also supports TCP/IP, so it's not a real problem...)