Fix code comment

The changes made in commit d2b4b4c225 contained incorrect comments:
They said that certain forward declarations were necessary to "avoid
including pathnodes.h here", but the file is itself pathnodes.h!  So
change the comment to just say it's a forward declaration in one case,
and in the other case we don't need the declaration at all because it
already appeared earlier in the file.

src/include/nodes/pathnodes.h

@@ -1134,8 +1134,7 @@ typedef struct IndexOptInfo IndexOptInfo;
 #define HAVE_INDEXOPTINFO_TYPEDEF 1
 #endif
 
-struct IndexPath;				/* avoid including pathnodes.h here */
-struct PlannerInfo;				/* avoid including pathnodes.h here */
+struct IndexPath;				/* forward declaration */
 
 struct IndexOptInfo
 {