nvme-pci: remove cq check after submission

We always check the completion queue after submitting, but in my testing
this isn't a win even on DRAM/xpoint devices. In some cases it's
actually worse. Kill it.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>