Back out the EXISTS-to-IN optimization. It slows things down rather than

speeds them up depending on the query.  And (see
[forum:/forumpost/8692d94725|forum post 8692d94725]) it sometimes results in
an incorrect answer.  We may come back and revisit this optimization later,
but for now it seems best just to disable it.

FossilOrigin-Name: 16252d73fa73569fd7506676f6ffbbcd43addfb105384fb74449d30ca720904a

src/sqliteInt.h

@@ -1731,7 +1731,6 @@ struct sqlite3 {
 #define SQLITE_SkipScan       0x00004000 /* Skip-scans */
 #define SQLITE_PropagateConst 0x00008000 /* The constant propagation opt */
 #define SQLITE_MinMaxOpt      0x00010000 /* The min/max optimization */
-#define SQLITE_ExistsToIN     0x00020000 /* The EXISTS-to-IN optimization */
 #define SQLITE_AllOpts        0xffffffff /* All optimizations */
 
 /*