[NETFILTER]: kill nf_sysctl.c

Since there now is generic support for shared sysctl paths, the only
remains are the net/netfilter and net/ipv4/netfilter paths. Move them
to net/netfilter/core.c and net/ipv4/netfilter.c and kill nf_sysctl.c.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>