Revert "Fix conversion to item_type on __call__ in List type"

This reverts commit 42af1ad0.
That commit breaks nova's test see:
http://logs.openstack.org/periodic/periodic-nova-py27-with-oslo-master/e52702e/testr_results.html.gz

Change-Id: If2747ede5da172431c53ab0f6d02a4c91d45d57c