Remove unused code for UNIQUE_PATH_LIMIT1.

The code that used to create these paths was removed in commit d4ce0921,
but this was left behind.

src/backend/cdb/cdbpath.c

@@ -1608,8 +1608,7 @@ cdbpath_dedup_fixup_unique(UniquePath *uniquePath, CdbpathDedupFixupContext *ctx
 		return CdbVisit_Walk;	/* onward to visit the kids */
 
 	/* No action needed if data is trivially unique. */
-	if (uniquePath->umethod == UNIQUE_PATH_NOOP ||
-		uniquePath->umethod == UNIQUE_PATH_LIMIT1)
+	if (uniquePath->umethod == UNIQUE_PATH_NOOP)
 		return CdbVisit_Walk;	/* onward to visit the kids */
 
 	/* Find set of relids for which subpath must produce row ids. */

src/backend/optimizer/plan/createplan.c

@@ -22,7 +22,6 @@
 #include <math.h>
 
 #include "catalog/pg_exttable.h"
-#include "catalog/pg_type.h"	/* INT8OID */
 #include "access/skey.h"
 #include "access/sysattr.h"
 #include "catalog/pg_class.h"
@@ -1052,26 +1051,6 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 	if (best_path->umethod == UNIQUE_PATH_NOOP)
 		return subplan;
 
-	/* CDB: Result will surely be unique if we produce only its first row. */
-	if (best_path->umethod == UNIQUE_PATH_LIMIT1)
-	{
-		Limit	   *limitplan;
-
-		/* Top off the subplan with a LIMIT 1 node. */
-		limitplan = makeNode(Limit);
-		copy_path_costsize(root, &limitplan->plan, &best_path->path);
-		limitplan->plan.targetlist = subplan->targetlist;
-		limitplan->plan.qual = NIL;
-		limitplan->plan.lefttree = subplan;
-		limitplan->plan.righttree = NULL;
-		limitplan->limitOffset = NULL;
-		limitplan->limitCount = (Node *) makeConst(INT8OID, -1, InvalidOid,
-												   sizeof(int64),
-												   Int64GetDatum(1),
-												   false, true);
-		return (Plan *) limitplan;
-	}
-
 	/*
 	 * As constructed, the subplan has a "flat" tlist containing just the Vars
 	 * needed here and at upper levels.  The values we are supposed to

src/include/nodes/relation.h

@@ -1234,8 +1234,7 @@ typedef enum
 {
 	UNIQUE_PATH_NOOP,			/* input is known unique already */
 	UNIQUE_PATH_HASH,			/* use hashing */
-	UNIQUE_PATH_SORT,			/* use sorting */
-    UNIQUE_PATH_LIMIT1          /* CDB: take at most one row from the subpath */
+	UNIQUE_PATH_SORT			/* use sorting */
 } UniquePathMethod;
 
 typedef struct UniquePath