netfilter: nfnetlink_queue: drop bogus WARN_ON

In the Linux kernel, the following vulnerability has been resolved: netfilter: nfnetlink_queue: drop bogus WARN_ON Happens when rules get flushed/deleted while packet is out, so remove this WARN_ON. This WARN exists in one form or another since v4.14, no need to backport this to older releases, hence use a more recent fixes tag.