Remove unnecessary forward declarations.

To match the upstream code.

src/include/executor/execdesc.h

@@ -22,8 +22,6 @@
 #include "gpmon/gpmon.h"
 
 struct CdbExplain_ShowStatCtx;  /* private, in "cdb/cdbexplain.c" */
-struct EState;                  /* #include "nodes/execnodes.h" */
-struct PlanState;               /* #include "nodes/execnodes.h" */
 
 
 /* GangType enumeration is used in several structures related to CDB
@@ -237,8 +235,8 @@ typedef struct QueryDesc
 
 	/* These fields are set by ExecutorStart */
 	TupleDesc	tupDesc;		/* descriptor for result tuples */
-	struct EState      *estate;			/* executor's query-wide state */
-	struct PlanState   *planstate;		/* tree of per-plan-node state */
+	EState	   *estate;			/* executor's query-wide state */
+	PlanState  *planstate;		/* tree of per-plan-node state */
 
 	/* This field is set by ExecutorEnd after collecting cdbdisp results */
 	uint64		es_processed;	/* # of tuples processed */

src/include/nodes/execnodes.h

@@ -29,7 +29,6 @@
  */
 #define InvalidPartitionSelectorId  0
 
-struct CdbDispatchResults;              /* in cdbdispatchresult.h */
 struct CdbExplain_ShowStatCtx;          /* private, in "cdb/cdbexplain.c" */
 struct ChunkTransportState;             /* #include "cdb/cdbinterconnect.h" */
 struct StringInfoData;                  /* #include "lib/stringinfo.h" */