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.

src/backend/access/heap/vacuumlazy.c

@@ -2478,13 +2478,6 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
  * index is an offset into the vacrel->dead_items array for the first listed
  * LP_DEAD item on the page.  The return value is the first index immediately
  * after all LP_DEAD items for the same page in the array.
- *
- * Prior to PostgreSQL 14 there were rare cases where this routine had to set
- * tuples with storage to unused.  These days it is strictly responsible for
- * marking LP_DEAD stub line pointers as unused.  This only happens for those
- * LP_DEAD items on the page that were determined to be LP_DEAD items back
- * when the same page was visited by lazy_scan_prune() (i.e. those whose TID
- * was recorded in the dead_items array at the time).
  */
 static int
 lazy_vacuum_heap_page(LVRelState *vacrel, BlockNumber blkno, Buffer buffer,