Temporary revert previous patch

Split previous patch in 2 steps: first remove
the MOVE_NULL hack, then retire nullChild.

The first step is a prerequisite
for second one and affects bench.

The second step (next patch) just removes nullChild
without affecting bench.

bench: 8205159

src/search.h

@@ -46,6 +46,7 @@ struct Stack {
   Depth reduction;
   Value staticEval;
   bool skipNullMove;
+  bool nullChild;
 };