Remove useless code in Position::pretty()

First, remove some dead code (function never called with a Move argument).

Then, remove printing of legal moves, which does not belong here. Let's keep commands orthogonal and minimal:
- the "d" command should display the board, nothing more, or less.
- "perft 1" will display the list of legal moves.

No functional change.

src/position.h

@@ -83,7 +83,7 @@ public:
   // Text input/output
   void set(const std::string& fenStr, bool isChess960, Thread* th);
   const std::string fen() const;
-  const std::string pretty(Move m = MOVE_NONE) const;
+  const std::string pretty() const;
 
   // Position representation
   Bitboard pieces() const;