netfilter: flowtable: remove hw_ifidx
hw_ifidx was originally introduced to store the real netdevice as a
requirement for the hardware offload support in:
73f97025a972 ("netfilter: nft_flow_offload: use direct xmit if hardware offload is enabled")
Since ("netfilter: flowtable: consolidate xmit path"), ifidx and
hw_ifidx points to the real device in the xmit path, remove it.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>