nvme: remove the ifdef around nvme_nvm_ioctl

We already have a proper stub if lightnvm is not enabled, so don't bother
with the ifdef.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>