Remove unused xl_heap_prune member, reason

f83d709760 refactored xl_heap_prune and added an unused member,
reason. While PruneReason is used when constructing this WAL record to
set the WAL record definition, it doesn't need to be stored in a
separate field in the record. Remove it.

We won't backport this, since modifying an exposed struct definition to
remove an unused field would do more harm than good.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reported-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>

Discussion: https://postgr.es/m/tvvtfoxz5ykpsctxjbzxg3nldnzfc7geplrt2z2s54pmgto27y%40hbijsndifu45

src/include/access/heapam_xlog.h

@@ -284,7 +284,6 @@ typedef struct xl_heap_update
  */
 typedef struct xl_heap_prune
 {
-	uint8		reason;
 	uint8		flags;
 
 	/*