Define OnePly as a Depth enum costant

There is no reason to use a variable for this.

Also remove unused DEPTH_ZERO and DEPTH_MAX.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/depth.h

@@ -26,19 +26,12 @@
 ////
 
 enum Depth {
-  DEPTH_ZERO = 0,
-  DEPTH_MAX = 200,  //  100 * OnePly;
-  DEPTH_NONE = -254 // -127 * OnePly
+
+  OnePly = 2,
+  DEPTH_NONE = -127 * OnePly
 };
 
 
-////
-//// Constants
-////
-
-const Depth OnePly = Depth(2);
-
-
 ////
 //// Inline functions
 ////