check_replace_refs: fix outdated comment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>

cache.h

@@ -804,9 +804,7 @@ void reset_shared_repository(void);
  * Do replace refs need to be checked this run?  This variable is
  * initialized to true unless --no-replace-object is used or
  * $GIT_NO_REPLACE_OBJECTS is set, but is set to false by some
- * commands that do not want replace references to be active.  As an
- * optimization it is also set to false if replace references have
- * been sought but there were none.
+ * commands that do not want replace references to be active.
  */
 extern int check_replace_refs;
 extern char *git_replace_ref_base;