Remove unneeded bitboard from MP

Recent simplification has removed the need for an extra bitboard in MP struct.
Use a local variable instead.

STC: Passed Non-regression test
https://tests.stockfishchess.org/tests/view/64294ae677ff3301150cba16
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 64872 W: 17383 L: 17203 D: 30286
Ptnml(0-2): 179, 6675, 18546, 6859, 177

closes https://github.com/official-stockfish/Stockfish/pull/4490

No functional change.

src/movepick.h

@@ -131,8 +131,6 @@ public:
   MovePicker(const Position&, Move, Value, const CapturePieceToHistory*);
   Move next_move(bool skipQuiets = false);
 
-  Bitboard threatenedPieces;
-
 private:
   template<PickType T, typename Pred> Move select(Pred);
   template<GenType> void score();