COMP: Remove unused variables

Windows build failed when making an unused variable constexpr.
Modules\Core\Common\test\itkVectorContainerTest.cxx(54): error C2127:
'p_null': illegal initialization of 'constexpr' entity with a
non-constant expression

Remove the unused variable to avoid the problem.

Modules/Core/Common/test/itkVectorContainerTest.cxx

@@ -33,10 +33,9 @@ itkVectorContainerTest(int, char *[])
 
   // Iterator
   {
-    [[maybe_unused]] constexpr ContainerType::Iterator p_null;
-    ContainerType::Iterator                            p = container->Begin();
-    ContainerType::Iterator                            p_copy(p);
-    ContainerType::Iterator                            p_assign = p;
+    ContainerType::Iterator p = container->Begin();
+    ContainerType::Iterator p_copy(p);
+    ContainerType::Iterator p_assign = p;
 
     while (p != container->End())
     {
@@ -51,10 +50,9 @@ itkVectorContainerTest(int, char *[])
 
   // ConstIterator
   {
-    [[maybe_unused]] constexpr ContainerType::ConstIterator p_null;
-    ContainerType::ConstIterator                            p = container->Begin();
-    ContainerType::ConstIterator                            p_copy(p);
-    ContainerType::ConstIterator                            p_assign = p;
+    ContainerType::ConstIterator p = container->Begin();
+    ContainerType::ConstIterator p_copy(p);
+    ContainerType::ConstIterator p_assign = p;
 
     while (p != container->End())
     {