netfilter: nft_set_pipapo: remove dirty flag

After previous change:
 ->clone exists: ->dirty is always true
 ->clone == NULL ->dirty is always false

So remove this flag.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>