nvme-core: remove redundant check from nvme_init_ns_head

nvme_find_ns_head already checks that the list of namescpaces
in an already existing namespace head is not empty

Signed-off-by: Irvin Cote <irvincoteg@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>