Revert "Don't prune discovered checks"

Both Lucas re-test and Jean-Francois confirrm it
is a regression.

Here Jean-Francois's results after 3600 games :
Score of 96d3b1c92b vs 3b87314: 690 - 729 - 2181 [0.495] 3600
ELO: -3.86 +- 99%: 14.94 95%: 11.35
LOS: 15.03%
Wins: 690 Losses: 729 Draws: 2181 Total: 3600

Bench: 5404066

src/types.h

@@ -130,12 +130,6 @@ enum MoveType {
   CASTLE    = 3 << 14
 };
 
-enum CheckType {
-  NO_CHECK,
-  DIRECT_CHECK,
-  DISCO_CHECK
-};
-
 enum CastleRight {  // Defined as in PolyGlot book hash key
   CASTLES_NONE = 0,
   WHITE_OO     = 1,