nvme-pci: Remove tag from process cq

The only user for tagged completion was for timeout handling. That user,
though, really only cares if the timed out command is completed, which
we can safely check within the timeout handler.

Remove the tag check to simplify completion handling.

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