Move ONE_PLY to be 1 instead of 2: search()

Now that half-plies are no more used we can simplify
the code assuming that ONE_PLY is 1 and no more 2.

Verified with a SMP test:
LLR: 2.95 (-2.94,2.94) [-4.50,0.00]
Total: 8926 W: 1712 L: 1607 D: 5607

No functional change.

src/types.h

@@ -211,14 +211,14 @@ enum Piece {
 
 enum Depth {
 
-  ONE_PLY = 2,
+  ONE_PLY = 1,
 
-  DEPTH_ZERO          =  0 * ONE_PLY,
-  DEPTH_QS_CHECKS     =  0 * ONE_PLY,
-  DEPTH_QS_NO_CHECKS  = -1 * ONE_PLY,
-  DEPTH_QS_RECAPTURES = -5 * ONE_PLY,
+  DEPTH_ZERO          =  0,
+  DEPTH_QS_CHECKS     =  0,
+  DEPTH_QS_NO_CHECKS  = -1,
+  DEPTH_QS_RECAPTURES = -5,
 
-  DEPTH_NONE = -6 * ONE_PLY
+  DEPTH_NONE = -6
 };
 
 enum Square {