check_replace_refs: fix outdated comment

check_replace_refs: fix outdated comment

Commit afc711b8e1 (rename read_replace_refs to
check_replace_refs, 2014-02-18) added a comment mentioning
that check_replace_refs is set in two ways:

  - from user intent via --no-replace-objects, etc

  - after seeing there are no replace refs to respect

Since c3c36d7de2 (replace-object: check_replace_refs is safe
in multi repo environment, 2018-04-11) the second is no
longer true. Let's drop that part of the comment.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>