Fix null reduction formula

Depth is already dependent on the actual value
of ONE_PLY, in particular can be expressed like:

Depth = n * ONE_PLY

And because formula is used to calculate R that is
also dependent on the value of ONE_PLY and can be
expressed like:

R = x * ONE_PLY

We don't want to divide depth by a 'ply' value but
directly by an integer number.

Spotted by sf-x

No functional change.

src/search.cpp

@@ -632,7 +632,7 @@ namespace {
 
         // Null move dynamic reduction based on depth and value
         Depth R =  3 * ONE_PLY
-                 + depth / (2 * ONE_PLY)
+                 + depth / 4
                  + int(eval - beta) / PawnValueMg * ONE_PLY;
 
         pos.do_null_move(st);