netfilter: nf_tables: fix underflow in object reference counter

Since ("netfilter: nf_tables: drop map element references from
preparation phase"), integration with commit protocol is better,
therefore drop the workaround that b91d9036

 ("netfilter: nf_tables:
fix leaking object reference count") provides.

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