Remove obsolete nbtree duplicate entries comment.

Remove a comment from the Berkeley days claiming that nbtree must
disambiguate duplicate keys within _bt_moveright().  There is no special
care taken around duplicates within _bt_moveright(), at least since
commit 9e85183b removed inscrutable _bt_moveright() code to handle
pages full of duplicates.

src/backend/access/nbtree/nbtinsert.c

@@ -814,10 +814,7 @@ _bt_findinsertloc(Relation rel,
  *		INCOMPLETE_SPLIT flag on it, and release the buffer.
  *
  *		The locking interactions in this code are critical.  You should
- *		grok Lehman and Yao's paper before making any changes.  In addition,
- *		you need to understand how we disambiguate duplicate keys in this
- *		implementation, in order to be able to find our location using
- *		L&Y "move right" operations.
+ *		grok Lehman and Yao's paper before making any changes.
  *----------
  */
 static void