Can't print PlannerGlobal's subroots list in outfuncs.

Since the subroots will surely link back to the same glob struct, this
necessarily leads to infinite recursion.  Doh.  Found while trying to
debug some other code.

src/backend/nodes/outfuncs.c

@@ -1654,7 +1654,6 @@ _outPlannerGlobal(StringInfo str, PlannerGlobal *node)
 	/* NB: this isn't a complete set of fields */
 	WRITE_NODE_FIELD(paramlist);
 	WRITE_NODE_FIELD(subplans);
-	WRITE_NODE_FIELD(subroots);
 	WRITE_BITMAPSET_FIELD(rewindPlanIDs);
 	WRITE_NODE_FIELD(finalrtable);
 	WRITE_NODE_FIELD(finalrowmarks);