nvmet: revert "nvmet: make discovery NQN configurable"

Revert commit 626851e9 ("nvmet: make discovery NQN configurable");
the interface was deemed incorrect and will be replaced with a different
one.

Fixes: 626851e9

 ("nvmet: make discovery NQN configurable")
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>