Remove obsolete "hole in center of page" comment.

Remove obsolete "hole in center of page" comment.

A comment from the Berkeley days incorrectly claimed that the page
management code cares about the contents of the hole in the center of
the page (at least in the case of the left half of an nbtree page
split).  Commit 8fa30f906be added an addendum that stated that the
original comment was "probably obsolete".  It's definitely obsolete,
though, so remove the original comment plus the addendum.