netfilter: remove include/net/netfilter/nft_dup.h

include/net/netfilter/nft_dup.h was introduced in d877f071

 ("netfilter: nf_tables: add nft_dup expression")
but was never user since this date.

Furthermore, the only struct in this file is unused elsewhere.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>