xfrm: free not used XFRM_ESP_NO_TRAILER flag

After removal of Innova IPsec support from mlx5 driver, the last user
of this XFRM_ESP_NO_TRAILER was gone too. This means that we can safely
remove it as no other hardware is capable (or need) to remove ESP trailer.

Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>