STYLE: Remove diff variables from ConvertPixelBuffer (warning C26451)

These four local `diff` variables appeared unnecessary, after commit
f8e3d3b ("ERR: ++ replaced with +1, +2, etc.
to avoid possible compiler problems.", Bill Lorensen, 2002-09-03). Removing
them fixed a few Visual Studio 2019 16.11.21 Code Analysis warnings, saying:

> warning C26451: Arithmetic overflow: Using operator '+' on a 4 byte value and
> then casting the result to a 8 byte value. Cast the value to the wider type
> before calling operator '+' to avoid overflow (io.2).

Modules/IO/ImageBase/include/itkConvertPixelBuffer.hxx

@@ -274,18 +274,16 @@ ConvertPixelBuffer<InputPixelType, OutputPixelType, OutputConvertTraits>::Conver
     // http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html
     // NOTE: The scale factors are converted to whole numbers for
     // precision
-    ptrdiff_t        diff = inputNumberOfComponents - 4;
     InputPixelType * endInput = inputData + size * (size_t)inputNumberOfComponents;
     while (inputData != endInput)
     {
       double tempval = ((2125.0 * static_cast<double>(*inputData) + 7154.0 * static_cast<double>(*(inputData + 1)) +
                          0721.0 * static_cast<double>(*(inputData + 2))) /
                         10000.0) *
                        static_cast<double>(*(inputData + 3)) / maxAlpha;
-      inputData += 4;
       auto val = static_cast<OutputComponentType>(tempval);
       OutputConvertTraits::SetNthComponent(0, *outputData++, val);
-      inputData += diff;
+      inputData += inputNumberOfComponents;
     }
   }
 }
@@ -371,15 +369,13 @@ ConvertPixelBuffer<InputPixelType, OutputPixelType, OutputConvertTraits>::Conver
   // just skip the rest of the data
   else
   {
-    ptrdiff_t        diff = inputNumberOfComponents - 3;
     InputPixelType * endInput = inputData + size * (size_t)inputNumberOfComponents;
     while (inputData != endInput)
     {
       OutputConvertTraits::SetNthComponent(0, *outputData, static_cast<OutputComponentType>(*inputData));
       OutputConvertTraits::SetNthComponent(1, *outputData, static_cast<OutputComponentType>(*(inputData + 1)));
       OutputConvertTraits::SetNthComponent(2, *outputData, static_cast<OutputComponentType>(*(inputData + 2)));
-      inputData += 3;
-      inputData += diff;
+      inputData += inputNumberOfComponents;
       ++outputData;
     }
   }
@@ -475,16 +471,14 @@ ConvertPixelBuffer<InputPixelType, OutputPixelType, OutputConvertTraits>::Conver
   }
   else
   {
-    ptrdiff_t        diff = inputNumberOfComponents - 4;
     InputPixelType * endInput = inputData + size * (size_t)inputNumberOfComponents;
     while (inputData != endInput)
     {
       OutputConvertTraits::SetNthComponent(0, *outputData, static_cast<OutputComponentType>(*inputData));
       OutputConvertTraits::SetNthComponent(1, *outputData, static_cast<OutputComponentType>(*(inputData + 1)));
       OutputConvertTraits::SetNthComponent(2, *outputData, static_cast<OutputComponentType>(*(inputData + 2)));
       OutputConvertTraits::SetNthComponent(3, *outputData, static_cast<OutputComponentType>(*(inputData + 3)));
-      inputData += 4;
-      inputData += diff;
+      inputData += inputNumberOfComponents;
       ++outputData;
     }
   }
@@ -603,15 +597,13 @@ ConvertPixelBuffer<InputPixelType, OutputPixelType, OutputConvertTraits>::Conver
   OutputPixelType * outputData,
   size_t            size)
 {
-  ptrdiff_t        diff = inputNumberOfComponents - 2;
   InputPixelType * endInput = inputData + size * (size_t)inputNumberOfComponents;
 
   while (inputData != endInput)
   {
     OutputConvertTraits::SetNthComponent(0, *outputData, static_cast<OutputComponentType>(*inputData));
     OutputConvertTraits::SetNthComponent(1, *outputData, static_cast<OutputComponentType>(*(inputData + 1)));
-    inputData += 2;
-    inputData += diff;
+    inputData += inputNumberOfComponents;
     ++outputData;
   }
 }