DOC: Document itk::Indent class methods in header file only

Document `itk::Indent` class methods in header file only: Doxygen picks
the method documentation from the header files, and leaves the method
documentation empty if it does not find it there. So remove the method
documentation from the implementation file.

Modules/Core/Common/src/itkIndent.cxx

@@ -34,19 +34,12 @@ namespace itk
 {
 static constexpr char blanks[ITK_NUMBER_OF_BLANKS + 1] = "                                        ";
 
-/**
- * Instance creation.
- */
 Indent *
 Indent::New()
 {
   return new Self;
 }
 
-/**
- * Determine the next indentation level. Keep indenting by two until the
- * max of forty.
- */
 Indent
 Indent::GetNextIndent() const
 {
@@ -59,9 +52,6 @@ Indent::GetNextIndent() const
   return indent;
 }
 
-/**
- * Print out the indentation. Basically output a bunch of spaces.
- */
 std::ostream &
 operator<<(std::ostream & os, const Indent & ind)
 {