Remove misleading comment from pathnodes.h.

As of commit e5253fdc, it is no longer true that the leader always
executes the subplan of a Gather Merge node.  Remove comment to that
effect.

Back-patch to 11.

Discussion: https://postgr.es/m/CA%2BhUKGJEaZJYezXAOutuiWT%2BfxCA44%2BoKtVPAND2ubLiigR%3D-w%40mail.gmail.com

src/include/nodes/pathnodes.h

@@ -1465,8 +1465,7 @@ typedef struct GatherPath
 
 /*
  * GatherMergePath runs several copies of a plan in parallel and collects
- * the results, preserving their common sort order.  For gather merge, the
- * parallel leader always executes the plan too, so we don't need single_copy.
+ * the results, preserving their common sort order.
  */
 typedef struct GatherMergePath
 {