Fixed backup file behavior in check mode (#24899)

Fixes ansible/ansible#21523

lib/ansible/modules/system/cron.py

@@ -752,11 +752,10 @@ def main():
             res_args['diff'] = diff
 
     # retain the backup only if crontab or cron file have changed
-    if backup:
+    if backup and not module.check_mode:
         if changed:
             res_args['backup_file'] = backup_file
         else:
-            if not module.check_mode:
             os.unlink(backup_file)
 
     if cron_file: