target: remove TRANSPORT_DEFERRED_CMD state

We never check for this state, and it makes testing for a completed
state much harder given that it overrides the existing state.

Also remove the unused deferred_t_state which is related to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>