netfilter: nf_flow_table: remove union from flow_offload structure

Drivers do not have access to the flow_offload structure, hence remove
this union from this flow_offload object as well as the original comment
on top of it.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>