netfilter: nf_tables: remove busy mark and gc batch API

commit a2dd0233

 upstream.

Ditch it, it has been replace it by the GC transaction API and it has no
clients anymore.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>