show-ref: remove a stale comment

show-ref: remove a stale comment

When cf0adba788 ("Store peeled refs in packed-refs file.",
2006-11-19) made the command to die with a message on error even
when --quiet is passed, it left the comment to say it changed the
semantics.  But that kind of information belongs to the log message,
not in-code comment.  Besides, the behaviour after the change has
been the established one for the past 10 years ;-)

Signed-off-by: Junio C Hamano <gitster@pobox.com>