netfilter: nf_tables: remove leftover set_update_list

This list has been moved to per-netns, remove onstack list which is not
used anymore.

Fixes: b343ededb3f9 ("netfilter: nf_tables: move set_update_list to nftables per-netns")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>