Remove obsolete comment atop ri_PlanCheck.

Commit 5b7ba75f removed the unused parameter but forgot to update the
nearby comments.

Author: Li Japin
Backpatch-through: 13, where it was introduced
Discussion: https://postgr.es/m/0E2F62A2-B2F1-4052-83AE-F0BEC8A75789@hotmail.com

src/backend/utils/adt/ri_triggers.c

@@ -2130,9 +2130,6 @@ InvalidateConstraintCacheCallBack(Datum arg, int cacheid, uint32 hashvalue)
 
 /*
  * Prepare execution plan for a query to enforce an RI restriction
- *
- * If cache_plan is true, the plan is saved into our plan hashtable
- * so that we don't need to plan it again.
  */
 static SPIPlanPtr
 ri_PlanCheck(const char *querystr, int nargs, Oid *argtypes,