cleanup of SmartRef

• remove redundant duplication of SmartRef constructor from {non-,}const pointer (which can only result in ambiguous overloads...)
• add final
• add some const
• prefer nullptr over 0

GaudiKernel/include/GaudiKernel/SmartRef.h

@@ -97,17 +97,10 @@ public:
     _setEnvironment( 0, 0 );
   }
   /// Standard Constructor with initialisation
-  SmartRef( TYPE* pObject ) {
-    m_base.m_hintID = INVALID;
-    m_base.m_linkID = INVALID;
-    m_target        = pObject;
-    _setEnvironment( 0, 0 );
-  }
-  /// Standard Constructor with initialisation from const object
   SmartRef( const TYPE* pObject ) {
     m_base.m_hintID = INVALID;
     m_base.m_linkID = INVALID;
-    m_target        = const_cast<TYPE*>( pObject );
+    m_target        = pObject;
     _setEnvironment( 0, 0 );
   }
   /// Copy Constructor