Remove unused message from iogrp_data setup

The iogrp_data is initialized during do_setup loaded.
The vdisk_count is originally used but later removed.
In ValueException raised, the variable is no longer required.
Remove this will give a clearer message when 
ValueException raised.

Change-Id: Ib7b6c6a870ddd7932e8da1436d615f9ee458bd93
Closes-Bug: #1266321
Merge "Remove unused message from iogrp_data setup"