Remove 'si' StateInfo variable/parameter.

Since st is a member of position we don't need to pass it separately as
parameter.

While being there also remove some line in pos_is_ok, where
a copy of StateInfo was made by using default copy constructor and
then verified it's correctedness by doing a memcmp.
There is no point in doing that.

Passed non-regression test
https://tests.stockfishchess.org/tests/view/64098d562644b62c33942b35
LLR: 3.24 (-2.94,2.94) <-1.75,0.25>
Total: 548960 W: 145834 L: 146134 D: 256992
Ptnml(0-2): 1617, 57652, 156261, 57314, 1636

closes https://github.com/official-stockfish/Stockfish/pull/4444

No functional change

src/position.h

@@ -179,8 +179,8 @@ public:
 private:
   // Initialization helpers (used while setting up a position)
   void set_castling_right(Color c, Square rfrom);
-  void set_state(StateInfo* si) const;
-  void set_check_info(StateInfo* si) const;
+  void set_state() const;
+  void set_check_info() const;
 
   // Other helpers
   void move_piece(Square from, Square to);