xtables-compat: remove useless functions

xtables-compat: remove useless functions

The static function nft_rule_list_get was exposed outside nft.c through
the nft_rule_list_create function, but this was never used out there.

A similar situation occurs with nftnl_rule_list_free and
nft_rule_list_destroy.

This patch removes nft_rule_list_create and nft_rule_list_destroy for
the sake of simplicity.

Signed-off-by: Pablo M. Bermudo Garay <pablombg@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>