Remove comment about historic heap vacuuming issue.

Remove comment about historic heap vacuuming issue.

Remove comment block about how heap page vacuuming used to set tuples
with storage to LP_UNUSED in a rare edge case that can no longer happen
following commit 8523492d4e.  The comments seem unnecessary now, since
it's now generally clear that heap vacuuming only applies to LP_DEAD
items from VACUUM's first heap pass following more recent work from
commits 12b5ade902 and 4f8d9d1217.