Partially revert "hw/block/nvme: drain namespaces on sq deletion"

This partially reverts commit 98f84f5a.

Since all "multi aio" commands are now reimplemented to properly track
the nested aiocbs, we can revert the "hack" that was introduced to make
sure all requests we're properly drained upon sq deletion.

The revert is partial since we keep the assert that no outstanding
requests remain on the submission queue after the explicit cancellation.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>