C comment: remove mention of use of t_hoff WAL structure member

Reported-by: Antonin Houska

Discussion: https://postgr.es/m/21643.1595353537@antos

Backpatch-through: 9.5

src/include/access/heapam_xlog.h

@@ -137,8 +137,6 @@ typedef struct xl_heap_truncate
  * or updated tuple in WAL; we can save a few bytes by reconstructing the
  * fields that are available elsewhere in the WAL record, or perhaps just
  * plain needn't be reconstructed.  These are the fields we must store.
- * NOTE: t_hoff could be recomputed, but we may as well store it because
- * it will come for free due to alignment considerations.
  */
 typedef struct xl_heap_header
 {