remove long broken SCSI to NVMe translations

[Keith Busch]

On Wed, Jun 14, 2017 at 08:35:35AM +0200, Christoph Hellwig wrote:
> TEST UNIT READY wa broken for a year and a half, and FORMAT UNIT and
> WRITE BUFFER for two years.  Instead of fixing them up remove them - no
> one should be sending SCSI commands to non-scsi devices anyway.

This is a step in the right direction, but I don't think it goes far
enough. Can we remove the whole thing now?