Retire mate threat extension

It seems we have a lot of totally useless code !

After 8577 games 1504 - 1451 - 5622 ELO +2 (+- 4.4)

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

src/position.h

@@ -231,9 +231,6 @@ public:
   bool is_mate() const;
   bool is_draw() const;
 
-  // Check if side to move could be mated in one
-  bool has_mate_threat();
-
   // Number of plies from starting position
   int startpos_ply_counter() const;