BUG: Remove redundant m_Threader shadow in MatchCardinalityImageToImageMetric

MatchCardinalityImageToImageMetric redeclared m_Threader that
shadows ImageToImageMetric's protected m_Threader. The base's
constructor already initializes m_Threader to MultiThreaderBase::New()
identical to the subclass's intended default, so the shadow was
purely redundant. Update GetMultiThreader to qualify with this->
and rely on the inherited member.

Author: hjmjohnson

Modules/Registration/Common/include/itkMatchCardinalityImageToImageMetric.h

@@ -130,7 +130,7 @@ class ITK_TEMPLATE_EXPORT MatchCardinalityImageToImageMetric : public ImageToIma
   MultiThreaderBase *
   GetMultiThreader()
   {
-    return m_Threader;
+    return this->m_Threader;
   }
 
 protected:
@@ -183,10 +183,6 @@ class ITK_TEMPLATE_EXPORT MatchCardinalityImageToImageMetric : public ImageToIma
   bool                       m_MeasureMatches{ true };
   std::vector<MeasureType>   m_ThreadMatches{};
   std::vector<SizeValueType> m_ThreadCounts{};
-
-  /** Support processing data in multiple threads. Used by subclasses
-   * (e.g., ImageSource). */
-  MultiThreaderBase::Pointer m_Threader{ MultiThreaderBase::New() };
 };
 } // end namespace itk