Retire 'finished' from MovePicker

It is not useful becasue it is safe to call
get_next_move() multiple times when phase == PH_STOP

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/movepick.h

@@ -64,7 +64,6 @@ private:
   const Position& pos;
   const History& H;
   MoveStack ttMoves[2], killers[2];
-  bool finished;
   int phase;
   const uint8_t* phasePtr;
   MoveStack *curMove, *lastMove, *lastBadCapture;