Retire mate threat detection from evaluation

Remove a lot of complex, obscure and useless code.

After 999 games at 1+0
Mod vs Orig +162 =673 -164 -1 ELO

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

src/evaluate.h

@@ -89,9 +89,6 @@ struct EvalInfo {
   // 2 to kingAdjacentZoneAttacksCount[BLACK].
   int kingAdjacentZoneAttacksCount[2];
 
-  // mateThreat[color] is a move for the given side which gives a direct mate.
-  Move mateThreat[2];
-
   // Middle game and endgame mobility scores
   Score mobility;