nvme/scsi: Remove START STOP emulation

Now that the broken power state control is gone, it appears to serve
no purpose.  Just delete it.  NVME devices don't have a concept of
started vs stopped anyway.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>