xfrm: drop not needed flags variable in XFRM offload struct

After drivers were converted to rely on direction, the flags is not
used anymore and can be removed.

Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>