Introduce captured_piece()

It will be used by future patches and also rearranges some
half cooked code that mistakenly ended up in master in the
past.

No functional change.

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

src/position.cpp

@@ -1301,11 +1301,6 @@ void Position::undo_null_move() {
 }
 
 
-///
-PieceType Position::captured_piece() const {
-  return st->capture;
-}
-
 /// Position::see() is a static exchange evaluator: It tries to estimate the
 /// material gain or loss resulting from a move. There are three versions of
 /// this function: One which takes a destination square as input, one takes a