COMP: No longer delete (= delete) PreFillDirection() function template

Pull request #2869
commit af32a77 ("COMP: Remove
unreachable code") "deleted" the `PreFillDirection()` function template
definition, by `= delete`. This appeared to cause compilation errors on
the following sites/builds:

  Site: RogueResearch7
  Build Name: Mac10.11-AppleClang-rel-x86_64-static
  Build Name: Mac10.11-AppleClang-dbg-x86_64-static

As reported by Jon Haitz Legarreta Gorroño at
#2869 (comment)

The error messages included:

    /Users/builder/externalModules/IO/NIFTI/test/itkNiftiImageIOTest.cxx:26:1: error: redefinition of 'PreFillDirection'
    PreFillDirection<1>()

The compilation error appears caused by Clang compiler issue "Bug 17537
- Explicit specialization of deleted function template not possible"
(https://bugs.llvm.org/show_bug.cgi?id=17537), reported by
jonathan.sauer, 2013-10-10, and fixed with Clang 3.9.

Worked around this compiler bug by removing `= delete`. Also removed the
declarations of the four `PreFillDirection()` specializations, as
suggested by Lee Newberg.

Modules/IO/NIFTI/test/itkNiftiImageIOTest.h

@@ -54,23 +54,10 @@ constexpr unsigned char LEFT = 128;    /*Bit pattern 1 0 0  00000*/
 constexpr unsigned char ANTERIOR = 64; /*Bit pattern 0 1 0  00000*/
 constexpr unsigned char SUPERIOR = 32; /*Bit pattern 0 0 1  00000*/
 
-// The only specializations of this function template that may be called are given below.
+// Specializations of this function template are only implemented for 1D to 4D (defined in the cxx file).
 template <unsigned int TDimension>
 typename itk::ImageBase<TDimension>::DirectionType
-PreFillDirection() = delete;
-
-template <>
-itk::ImageBase<1>::DirectionType
-PreFillDirection<1>();
-template <>
-itk::ImageBase<2>::DirectionType
-PreFillDirection<2>();
-template <>
-itk::ImageBase<3>::DirectionType
-PreFillDirection<3>();
-template <>
-itk::ImageBase<4>::DirectionType
-PreFillDirection<4>();
+PreFillDirection();
 
 template <typename T>
 int