Remove unused variable from pqMultiBlockInspectorPanel

This removes the unused pqTreeWidgetSelectionHelper variable from
the constructor of the pqMultiBlockInspectorPanel class.

Change-Id: I40b467544cbdca63a84f40ae46298c4f10e3d8cc
Merge topic 'composite-data-display-attributes' into next

0bf3865 Remove unused variable from pqMultiBlockInspectorPanel