Unused private function 'initializeNULLNode' in RedBlackTree.cpp

[NiekBeijloos]

https://github.com/Bibeknam/algorithmtutorprograms/blob/bbbc4ff16b15a2fd50308c996e236d124a024b85/data-structures/red-black-trees/RedBlackTree.cpp#L28C1-L28C1