Fix nbtree backward scan race condition comments.

Fix nbtree backward scan race condition comments.

Remove comments that supposed that holding a pin was a useful interlock
for _bt_walk_left().  There are times when _bt_walk_left() doesn't hold
either a lock or a pin on any page, so clearly this can't be true.
_bt_walk_left() is even prepared to deal with concurrent deletion of
both the original page and any pages to its left.

Oversight in commit 2ed5b87f96.