Remove net.link.ether.inet.useloopback sysctl tunable. It was always on by

Remove net.link.ether.inet.useloopback sysctl tunable. It was always on by
default from the very beginning. It was placed in wrong namespace
net.link.ether, originally it had been at another wrong namespace. It was
incorrectly documented at incorrect manual page arp(8). Since new-ARP commit,
the tunable have been consulted only on route addition, and ignored on route
deletion. Behaviour of a system with tunable turned off is not fully correct,
and has no advantages comparing to normal behavior.