remove StateInfo::next

unused.

closes https://github.com/official-stockfish/Stockfish/pull/5997

no functional change

src/position.cpp

@@ -698,7 +698,6 @@ DirtyPiece Position::do_move(Move                      m,
     // our state pointer to point to the new (ready to be updated) state.
     std::memcpy(&newSt, st, offsetof(StateInfo, key));
     newSt.previous = st;
-    st->next       = &newSt;
     st             = &newSt;
 
     // Increment ply counters. In particular, rule50 will be reset to zero later on
@@ -1011,7 +1010,6 @@ void Position::do_null_move(StateInfo& newSt, const TranspositionTable& tt) {
     std::memcpy(&newSt, st, sizeof(StateInfo));
 
     newSt.previous = st;
-    st->next       = &newSt;
     st             = &newSt;
 
     if (st->epSquare != SQ_NONE)

src/position.h

@@ -53,7 +53,6 @@ struct StateInfo {
     Key        key;
     Bitboard   checkersBB;
     StateInfo* previous;
-    StateInfo* next;
     Bitboard   blockersForKing[COLOR_NB];
     Bitboard   pinners[COLOR_NB];
     Bitboard   checkSquares[PIECE_TYPE_NB];
@@ -165,7 +164,6 @@ class Position {
     bool pos_is_ok() const;
     void flip();
 
-    // Used by NNUE
     StateInfo* state() const;
 
     void put_piece(Piece pc, Square s);