Recent changes ( 2fe5a79425c7 ) pushed RTA_DST masking from route control code to the rtsock code.
It broke arp/ndp deletion.
It turns out, arp/ndp perform `RTM_GET` request first to gen an interface index, then simply stamp it with `RTF_LLDATA` and update the command to be `RTM_DELETE`.
As the kernel has always ignored pretty much everything except `RTA_DST`, `RTA_GATEWAY` and `RTF_ANNOUNCE`, it worked.
Fix this by removing `RTA_NETMASK` for `RTF_LLINFO` messages in the early phase.
While here, cleanup some unused code from arp/ndp (proper fix to handle RTM_DELETE will be done separately).