netlink: cleanup rntl_af_register

The function __rtnl_af_register is never called outside this
code, and the return value is always 0.

Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/rtnetlink.h
Simple merge

diff --cc net/bridge/br_netlink.c
Simple merge

diff --cc net/core/rtnetlink.c
Simple merge

diff --cc net/ipv6/addrconf.c
Simple merge