Remove obsolete nbtree duplicate entries comment.

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 9e85183bfc3 removed inscrutable _bt_moveright() code to handle
pages full of duplicates.