nvme-pci: remove SGL segment descriptors

The max segments this driver can see is 127, well below the 256
threshold needed to add an nvme sgl segment descriptor. Remove all the
useless checks and dead code.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>