netfilter: nf_flow_table: remove unnecessary nat flag check code

nf_flow_offload_{ip/ipv6}_hook() check nat flag then, call
nf_flow_nat_{ip/ipv6} but that also check nat flag. so that
nat flag check code in nf_flow_offload_{ip/ipv6}_hook() are unnecessary.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>