BUG: Fixed SubjectHierarchyGenericSelfTest
Fixes #3754

Additional minor fixes in handling potential nodes in the subject hierarchy tree (removed unavailable context menu items)
Removed potential subject hierarchy node list and related classes (model, proxy model, designer plugin)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23505 3bd1e089-480b-0410-8dfb-8563597acbee