Retire SearchStack init() and initKillers()

Let be explicit about what this functions do, and
we save some code lines too.

No functional change.

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

src/search.h

@@ -57,9 +57,6 @@ struct SearchStack {
   Depth reduction;
   Value eval;
   bool skipNullMove;
-
-  void init();
-  void initKillers();
 };