Update obsolete nbtree deletion comments.

Update obsolete nbtree deletion comments.

_bt_delitems_delete() is no longer the high-level entry point used by
index tuple deletion driven by index tuples whose LP_DEAD bits are set
(now called "simple index tuple deletion").  It became a lower level
routine that's only called by _bt_delitems_delete_check() following
commit d168b66682.