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>

include/net/netfilter/nft_dup.h

-/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef _NFT_DUP_H_
-#define _NFT_DUP_H_
-
-struct nft_dup_inet {
-	enum nft_registers	sreg_addr:8;
-	enum nft_registers	sreg_dev:8;
-};
-
-#endif /* _NFT_DUP_H_ */