BUG: Delete unnecessary Q_ASSERT in unit aware widgets

These asserts were added with the slicer normal context of execution in
 mind. In this case a scene should have a selection node at all time.
However, in tests that is not the case. Since the code is already
acounting for the fact that there might not be a selection node
(see updateWidgetFromUnitNode()), the asserts can just be removed.

See Issue #3143

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22078 3bd1e089-480b-0410-8dfb-8563597acbee