Tasks: fix remove_orphan_build_config
A limit/offset can't be used when deleting objects,
see https://read-the-docs.sentry.io/issues/7223445833/?alert_rule_id=229005&alert_timestamp=1769567444834&alert_type=email&environment=production¬ification_uuid=6572ff50-6152-4731-92f0-1c59543a307a&project=148442&referrer=alert_email.
If we want to use a limit, we can use the function from https://github.com/readthedocs/readthedocs.org/pull/12732/changes,
but we shouldn't have that many orphan build configs to delete at once.
