netfilter: nat: use test_and_clear_bit when deleting ct from bysource list

We can use a single statement for this.
While at it, fixup the comment -- we don't have pernet table/ops
anymore, the function is only called from module exit path.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>