[Backport release-1.2] fix(mariadb): remove duplicate template key in backup CronJob (#2652)

## Summary

Minimal backport from #2279 to `release-1.2` — only the
duplicate-`template:` key removal in
`packages/apps/mariadb/templates/backup-cronjob.yaml`, without the
breaking service-naming / forced-replication changes from #2279 (those
are not appropriate for a patch release).

Fixes the `yaml: unmarshal errors: line 17: mapping key "template"
already defined at line 14` error when deploying MariaDB with
`backup.enabled: true` on v1.2.x.

Related: #2293

## Test plan
- [ ] `helm template` of the mariadb chart with `backup.enabled: true`
renders without YAML duplicate-key errors
- [ ] CronJob deploys successfully on a v1.2.x cluster

packages/apps/mariadb/templates/backup-cronjob.yaml

@@ -13,9 +13,6 @@ spec:
   jobTemplate:
     spec:
       backoffLimit: 2
-      template:
-        spec:
-          restartPolicy: OnFailure
       template:
         metadata:
           annotations: