Unify undo_promotion_move()

Integrate do_ep_move in undo_move() this reduces line count
and code readibility.

No functional change.

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

src/position.h

@@ -313,7 +313,6 @@ private:
   void do_capture_move(PieceType capture, Color them, Square to, bool ep);
   void do_castle_move(Move m);
   void undo_castle_move(Move m);
-  void undo_promotion_move(Move m);
   void undo_ep_move(Move m);
   void find_checkers();