netfilter: nf_tables: do not remove elements if set backend implements .abort

pipapo set backend maintains two copies of the datastructure, removing
the elements from the copy that is going to be discarded slows down
the abort path significantly, from several minutes to few seconds after
this patch.

Fixes: 212ed75d

 ("netfilter: nf_tables: integrate pipapo into commit protocol")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>