src: add nft_*_list_del

This allows us to delete object from the list, note that
nft_chain_list_del already existed.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
rule: don't put NFTA_RULE_EXPRESSION into messages

If there are no expressions in this rule, skip the
NFTA_RULE_EXPRESSION attribute, otherwise it will be
empty.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>