Revert "Do execGrouping.c via expression eval machinery."

This reverts commit 773aec7aa9.

There's an unresolved issue in the reverted commit: It only creates
one comparator function, but in for the nodeSubplan.c case we need
more (c.f. FindTupleHashEntry vs LookupTupleHashEntry calls in
nodeSubplan.c).

This isn't too difficult to fix, but it's not entirely trivial
either. The fact that the issue only causes breakage on 32bit systems
shows that the current test coverage isn't that great.  To avoid
turning half the buildfarm red till those two issues are addressed,
revert.

src/include/executor/execExpr.h

@@ -148,7 +148,6 @@ typedef enum ExprEvalOp
 	/* evaluate assorted special-purpose expression types */
 	EEOP_IOCOERCE,
 	EEOP_DISTINCT,
-	EEOP_NOT_DISTINCT,
 	EEOP_NULLIF,
 	EEOP_SQLVALUEFUNCTION,
 	EEOP_CURRENTOFEXPR,