netfilter: nft_set_hash: remove nft_hash_key()

hashtable is never used for 2-byte keys, remove nft_hash_key().

Fixes: e240cd0d ("netfilter: nf_tables: place all set backends in one single module")
Reported-by: Florian Westphal <fw@strlen.de>
Tested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>