copr-keygen: backup: remove correct file

And produce no stderr in cronjob to avoid unnecessary emails sent
(hourly) by cron.

roles/copr/keygen/files/backup_keyring.sh

@@ -15,9 +15,6 @@ tar --exclude="*agent*" -czPf - "$PATH_TO_KEYRING_DIR" \
 # shell pattern matching provides sorted output
 previous=
 for file in "$BACKUP_DIR"/*; do
-    if test -n "$previous"; then
-        echo >&2 "removing $file"
-        rm "$file"
-    fi
+    test -z "$previous" || rm "$previous"
     previous=$file
 done