COMP: fixed various Wunused-but-set-variable warnings from Clang 13

Two solutions:
- removed the dead code.
- use the variable by logging it

Modules/Core/Common/test/itkPeriodicBoundaryConditionTest.cxx

@@ -302,16 +302,17 @@ itkPeriodicBoundaryConditionTest(int, char *[])
   back.push_back({ { 0, 0 } });
   testIter.SetNeedToUseBoundaryCondition(true);
   testIter.GoToBegin();
+  float sum = 0.0f;
   while (!testIter.IsAtEnd())
   {
-    float sum = 0;
     for (auto & j : back)
     {
       const float d = testIter.GetPixel(j);
-      sum += d * d;
+      sum += d;
     }
     ++testIter;
   }
+  std::cout << "Dummy sum: " << sum << std::endl;
 
   return EXIT_SUCCESS;
 }

Modules/Core/Common/test/itkTimeProbeTest.cxx

@@ -46,6 +46,7 @@ itkTimeProbeTest(int, char *[])
   {
     sum += i;
   }
+  std::cout << "Dummy sum: " << sum << std::endl;
   localTimer.Stop();
 
   // Print current values

Modules/Core/Common/test/itkTimeProbeTest2.cxx

@@ -74,6 +74,7 @@ itkTimeProbeTest2(int, char *[])
     {
       sum += i;
     }
+    std::cout << "Dummy sum: " << sum << std::endl;
 
     localTimer.Stop();
   }

Modules/IO/MRC/src/itkMRCHeaderObject.cxx

@@ -155,6 +155,9 @@ MRCHeaderObject::SetHeader(const Header * buffer)
   {
     extendedHeaderBytes += 2;
   }
+  // TODO: all the above to set extendedHeaderBytes, yet extendedHeaderBytes is unused!
+  itkWarningMacro(<< "extendedHeaderBytes is: " << extendedHeaderBytes <<
+    "If you see this log please contact https://github.com/InsightSoftwareConsortium/ITK/pull/2778");
 
   this->m_ExtendedHeaderSize = this->m_Header.next;
 

Modules/Segmentation/Voronoi/include/itkVoronoiSegmentationImageFilter.hxx

@@ -138,7 +138,6 @@ VoronoiSegmentationImageFilter<TInputImage, TOutputImage, TBinaryPriorImage>::Ta
   }
 
   float addb = 0;
-  float addbb = 0;
   int   numb = 0;
 
   ait.GoToBegin();
@@ -172,7 +171,6 @@ VoronoiSegmentationImageFilter<TInputImage, TOutputImage, TBinaryPriorImage>::Ta
         numb++;
         currp = (float)(iit.Get());
         addb += currp;
-        addbb += currp * currp;
       }
       ++ait;
       ++iit;