selftests: mptcp: stop forcing iptables-legacy

Commit 0c4cd3f8 ("selftests: mptcp: join: use 'iptables-legacy' if
available") and commit a5a5990c ("selftests: mptcp: sockopt: use
'iptables-legacy' if available") forced using iptables-legacy if
available.

This was needed because of some issues that were visible when testing
the kselftests on a v5.15.x with iptables-nft as default backend. It
looks like these errors are no longer present. As mentioned by Pablo [1],
the errors were maybe due to missing kernel config. We can then use
iptables-nft if it is the default one, instead of using a legacy tool.

We can then check the variables iptables and ip6tables are valid. We can
keep the variables to easily change it later or add options.

Link: https://lore.kernel.org/netdev/ZbFiixyMFpQnxzCH@calendula/

 [1]
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-1-bc79e6e5e6a0@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>