Remove duplicated comment block in event_trigger.c

The reasons why event triggers are disabled in standalone mode are
documented in the code path of ddl_command_start, and other places
checking if standalone mode is enabled or not mention to refer to the
comment for ddl_command_start, except for table_rewrite that duplicated
the same explanation.

Reported-by: David G. Johnston
Discussion: https://postgr.es/m/CAKFQuwYqHtXpvr2mBJRwH9f+Y5y1GXw3rhbaAu0Dk2MoNevsmA@mail.gmail.com

Author: michaelpq

src/backend/commands/event_trigger.c

@@ -833,20 +833,8 @@ EventTriggerTableRewrite(Node *parsetree, Oid tableOid, int reason)
 	EventTriggerData trigdata;
 
 	/*
-	 * Event Triggers are completely disabled in standalone mode.  There are
-	 * (at least) two reasons for this:
-	 *
-	 * 1. A sufficiently broken event trigger might not only render the
-	 * database unusable, but prevent disabling itself to fix the situation.
-	 * In this scenario, restarting in standalone mode provides an escape
-	 * hatch.
-	 *
-	 * 2. BuildEventTriggerCache relies on systable_beginscan_ordered, and
-	 * therefore will malfunction if pg_event_trigger's indexes are damaged.
-	 * To allow recovery from a damaged index, we need some operating mode
-	 * wherein event triggers are disabled.  (Or we could implement
-	 * heapscan-and-sort logic for that case, but having disaster recovery
-	 * scenarios depend on code that's otherwise untested isn't appetizing.)
+	 * See EventTriggerDDLCommandStart for a discussion about why event
+	 * triggers are disabled in single user mode.
 	 */
 	if (!IsUnderPostmaster)
 		return;