Revert "netfilter: nf_tables: add flush field to struct nft_set_iter"

This reverts commit 1f48ff6c.

This patch is not required anymore now that we keep a dummy list of
set elements in the bitmap set implementation, so revert this before
we forget this code has no clients.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>