nvme-auth: remove redundant auth_work flush

only ctrl deletion calls nvme_auth_free, which was stopped prior in the
teardown stage, so there is no possibility that it should ever run when
nvme_auth_free is called. As a result, we can remove a local chap pointer
variable.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>