Remove a function we dont use any more
Add a PDU flag that when set will invoke the callback with CANCELLED
from the destructor.
Set it when we send a iscsi command carrying a scsi_task and clear
it when we have successfully received a reply and invoked the callback
normally.

If set in the destructor, then invoke the callb ack with status CANCELLED.

This allows us to simply talloc_free(task)  and if this task was still in flight, then the iscsi_pdu will be unlinked from the queues properly.

As such  cancelling a command that is in flight thus becomes a simple
talloc_free(task)