nvme/pci: Remove last_cq_head

We had been saving the last_cq_head seen from an interrupt so that a
polled queue wouldn't mistakenly trigger spruious interrupt detection. We
don't poll interrupt driven queues any more, so saving this value is
pointless.

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