[NETFILTER]: Mark old IPv4-only connection tracking scheduled for removal

Also remove the references to "new connection tracking" from Kconfig.
After some short stabilization period of the new connection tracking
helpers/NAT code the old one will be removed.

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