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

include/net/netfilter/nft_dup.h was introduced in d877f07 ("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 <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>

Author: montjoie