STYLE: Remove const overload of SubjectImplementation::AddObserver

`SubjectImplementation` is an internal (hidden) helper class of `itk::Object`,
which only uses the non-const overload of `SubjectImplementation::AddObserver`.

Removing this unused overload reduces maintenance.

Modules/Core/Common/src/itkObject.cxx

@@ -67,9 +67,6 @@ class ITKCommon_HIDDEN SubjectImplementation
   unsigned long
   AddObserver(const EventObject & event, Command * cmd);
 
-  unsigned long
-  AddObserver(const EventObject & event, Command * cmd) const;
-
   void
   RemoveObserver(unsigned long tag);
 
@@ -139,17 +136,6 @@ SubjectImplementation::AddObserver(const EventObject & event, Command * cmd)
   return ptr->m_Tag;
 }
 
-unsigned long
-SubjectImplementation::AddObserver(const EventObject & event, Command * cmd) const
-{
-  auto * ptr = new Observer(cmd, event.MakeObject(), m_Count);
-  auto * me = const_cast<SubjectImplementation *>(this);
-
-  me->m_Observers.push_back(ptr);
-  me->m_Count++;
-  return ptr->m_Tag;
-}
-
 void
 SubjectImplementation::RemoveObserver(unsigned long tag)
 {