BUG: Fixed RemoveAllChildren method in subject hierarchy

It deleted the scene's immediate children, but in course of deletion, ReparentChildrenToParent was called (which is a feature to prevent such deletions to result dangling orphan items in memory), so the children of the top-level node's children were added under the scene, which were then not deleted. This caused a memory leak

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25773 3bd1e089-480b-0410-8dfb-8563597acbee