refactor(backup): remove duplicate direct-to-S3 stage

A host-to-iDrive S3 stage was added here and is now removed. It would
have written the same data into the same `backup-all` bucket that the
TrueNAS cloud-sync task already fills -- duplicate storage, two writers
to one prefix, for no additional coverage.

Offsite to business-owned storage was already solved: the rsync feeds
/mnt/glacier/skudakcloud and TrueNAS cloud-syncs that to Skudak's own
iDrive e2 account. The earlier note in skudak/cloud.yml proposed adding
S3 *and then dropping the rsync* -- replacement, not addition -- and
building both was a misreading of it.

If offsite is ever moved onto this host it must REPLACE the rsync, not
run beside it. Settle first whether the TrueNAS -> iDrive leg is
independently verifiable; keeping this chain means trusting it.

Also removes the now-orphaned /etc/backup_s3 credential file.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ansible/roles/podman/templates/nextcloud/cloud-backup.sh.j2

@@ -171,5 +171,4 @@ log "syncing db dumps"
 rsync -az --timeout=600 --delete --mkpath {{ backup_rsync_extra_args | default('') }} \
   -e "$SSH" "$STAGE/db/" "$DEST:{{ remote_path }}/_backup/db/"
 {% endif %}
-
 log "status=ok"