netfilter: kconfig: remove ct zone/label dependencies

connection tracking zones currently depend on the xtables CT target.
The reasoning was that it makes no sense to support zones if they can't
be configured (which needed CT target).

Nowadays zones can also be used by OVS and configured via nftables,
so remove the dependency.

connection tracking labels are handled via hidden dependency that gets
auto-selected by the connlabel match.
Make it a visible knob, as labels can be attached via ctnetlink
or via nftables rules (nft_ct expression) too.

This allows to use conntrack labels and zones with nftables-only build.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>