nvme: unexport nvme_delete_ctrl_sync()

Since nvme_delete_ctrl_sync() is not called from any other kernel module,
unexport it.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>