Remove pawnKey from StateInfo

Remove pawnKey since it is not used anymore.

Passed Non-regression STC:
https://tests.stockfishchess.org/tests/view/64b023110cdec37b9573265c
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 334848 W: 85440 L: 85545 D: 163863
Ptnml(0-2): 1134, 38101, 89075, 37964, 1150

closes #4687

No functional change

src/position.cpp

@@ -336,7 +336,6 @@ void Position::set_check_info() const {
 void Position::set_state() const {
 
   st->key = st->materialKey = 0;
-  st->pawnKey = Zobrist::noPawns;
   st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
   st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
 
@@ -348,10 +347,7 @@ void Position::set_state() const {
       Piece pc = piece_on(s);
       st->key ^= Zobrist::psq[pc][s];
 
-      if (type_of(pc) == PAWN)
-          st->pawnKey ^= Zobrist::psq[pc][s];
-
-      else if (type_of(pc) != KING)
+      if (type_of(pc) != KING && type_of(pc) != PAWN)
           st->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
   }
 
@@ -745,8 +741,6 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
               assert(piece_on(to) == NO_PIECE);
               assert(piece_on(capsq) == make_piece(them, PAWN));
           }
-
-          st->pawnKey ^= Zobrist::psq[captured][capsq];
       }
       else
           st->nonPawnMaterial[them] -= PieceValue[MG][captured];
@@ -825,17 +819,13 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
 
           // Update hash keys
           k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
-          st->pawnKey ^= Zobrist::psq[pc][to];
           st->materialKey ^=  Zobrist::psq[promotion][pieceCount[promotion]-1]
                             ^ Zobrist::psq[pc][pieceCount[pc]];
 
           // Update material
           st->nonPawnMaterial[us] += PieceValue[MG][promotion];
       }
 
-      // Update pawn hash key
-      st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
-
       // Reset rule 50 draw counter
       st->rule50 = 0;
   }

src/position.h

@@ -40,7 +40,6 @@ namespace Stockfish {
 struct StateInfo {
 
   // Copied when making a move
-  Key    pawnKey;
   Key    materialKey;
   Value  nonPawnMaterial[COLOR_NB];
   int    castlingRights;
@@ -338,10 +337,6 @@ inline Key Position::adjust_key50(Key k) const
       ? k : k ^ make_key((st->rule50 - (14 - AfterMove)) / 8);
 }
 
-inline Key Position::pawn_key() const {
-  return st->pawnKey;
-}
-
 inline Key Position::material_key() const {
   return st->materialKey;
 }