vtkEqualizerContextItem: remove unused code

The member operator can be used if it is `const` qualified.

Interaction/Widgets/vtkEqualizerContextItem.cxx

@@ -57,15 +57,9 @@ struct EqualizerPoint
   }
 
   //------------------------------------------------------------------------------
-  bool operator<(const EqualizerPoint& point) { return this->freq < point.freq; }
+  bool operator<(const EqualizerPoint& point) const { return this->freq < point.freq; }
 };
 
-//------------------------------------------------------------------------------
-bool operator<(const EqualizerPoint& lhs, const EqualizerPoint& rhs)
-{
-  return lhs.freq < rhs.freq;
-}
-
 //------------------------------------------------------------------------------
 bool isNear(vtkVector2f pos1, vtkVector2f pos2, double radius)
 {