SEE: simplify stm variable initialization

Pull #3458 removed the only usage of pos.see_ge() moving pieces that
don't belong to the side to move, so we can simplify this.
see_ge is on the 'hot path' so it should be optimized.

no functional change
bench: 4687476

src/position.cpp

@@ -1081,7 +1081,7 @@ bool Position::see_ge(Move m, Value threshold) const {
       return true;
 
   Bitboard occupied = pieces() ^ from ^ to;
-  Color stm = color_of(piece_on(from));
+  Color stm = sideToMove;
   Bitboard attackers = attackers_to(to, occupied);
   Bitboard stmAttackers, bb;
   int res = 1;