pg_resetwal: Update an obsolete comment

The comment claimed that pg_resetwal updates the pg_control file if it
is of an old version.  This has apparently never been true.  Also, in
c3c09be34b, another comment was added elsewhere that this currently
does not happen.  So this comment is wrong and redundant and can be
removed.

Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://www.postgresql.org/message-id/flat/0f3ab4a1-ae80-56e8-3426-6b4a02507687@eisentraut.org

src/bin/pg_resetwal/pg_resetwal.c

@@ -6,8 +6,7 @@
  *
  * The theory of operation is fairly simple:
  *	  1. Read the existing pg_control (which will include the last
- *		 checkpoint record).  If it is an old format then update to
- *		 current format.
+ *		 checkpoint record).
  *	  2. If pg_control is corrupt, attempt to intuit reasonable values,
  *		 by scanning the old xlog if necessary.
  *	  3. Modify pg_control to reflect a "shutdown" state with a checkpoint