nvme: update comment to make the code easier to read

After commit a686ed75

 ("nvme: introduce a helper function for
controller deletion), nvme_delete_ctrl_sync no longer use flush_work.
Update comment, accordingly.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>