How to calculate the output impedance of this circuit?
This circuit model is a logical impossibility. 

The control current of the CCCS is the same as its output current. If the gain of the CCCS were anything other than 1, it would be obvious why this is a problem: a current can't be \$Z\$ times itself for any value of \$Z\$ other than 1. (except of course if the current happens to be 0). 

Setting the gain to 1 makes this logical contradiction seem to disappear, but the circuit model is still a poor one --- in any real circuit there'd be uncertainty about what that gain is exactly.

So, there's no more sense to trying to reason about this circuit than there is to contemplating the equation 

$$3 = 2\times{}5$$

Edit

The question has been clarified to indicate this is meant to be a simplification of the hybrid pi model of a BJT. Here's the usual hybrid pi model:

(source: Wikimedia)

The issue with your model is resolved by including the \$r_o\$ element.