Remove BGP localPref to support upgrade from 1.6 in CI

In https://gitlab.com/sylva-projects/sylva-core/-/jobs/14873525551 we can see

╰┄╴2026-06-16 12:22:00 (x20 over 19m30s)                                                           Warning UpgradeFailed (combined from similar events): Helm upgrade failed for release metallb-system/metallb-resources with chart metallb-resources@0.3.1: failed to create resource: admission webhook "bgpadvertisementvalidationwebhook.metallb.io" denied the request: invalid local preference 200: local preferernce 0 was already set for the same type of BGP update. Check existing BGP advertisements with common pools and aggregation lengths

because in 1.6 we don't specify it.