netfilter: nf_dup4: remove redundant checksum recalculation

IP header checksum will be recalculated at ip_local_out, so
there's no need to calculated it here, remove it. Also update
code comments to illustrate it, and delete the misleading
comments about checksum recalculation.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>