netfilter: nf_tables: remove nhooks field from struct nft_af_info

We already validate the hook through bitmask, so this check is
superfluous. When removing this, this patch is also fixing a bug in the
new flowtable codebase, since ctx->afi points to the table family
instead of the netdev family which is where the flowtable is really
hooked in.

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