Rename promotion_piece_type() to promotion_type()

Shorter and equally clear to understand.

No functional change.

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

src/movepick.cpp

@@ -167,7 +167,7 @@ void MovePicker::score_captures() {
                   - type_of(pos.piece_moved(m));
 
       if (is_promotion(m))
-          cur->score += PieceValueMidgame[promotion_piece_type(m)];
+          cur->score += PieceValueMidgame[promotion_type(m)];
   }
 }