Remove obsolete comment in reorderbuffer.h.

Commit 9fab40ad32 changed ReorderBuffer to use Slab Context for
allocating ReorderBufferTXN entries instead of using a caching
mechanism. The txn->node is no longer used as an element of the list
of preallocated ReorderBufferTXNs.

Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CAD21AoB1CTnX66Ji3zTCnjoPVC9OzYe0B6LygUHcxEB2RV-hFw%40mail.gmail.com

src/include/replication/reorderbuffer.h

@@ -394,10 +394,9 @@ typedef struct ReorderBufferTXN
 	SharedInvalidationMessage *invalidations;
 
 	/* ---
-	 * Position in one of three lists:
+	 * Position in one of two lists:
 	 * * list of subtransactions if we are *known* to be subxact
 	 * * list of toplevel xacts (can be an as-yet unknown subxact)
-	 * * list of preallocated ReorderBufferTXNs (if unused)
 	 * ---
 	 */
 	dlist_node	node;