Move deleting namespace children logic to a dedicated task

[kyasbal]

Problem

Current k8s audit log parser implementation handles the delete collection api call for namespace in timeline grouping task.
https://github.com/GoogleCloudPlatform/khi/blob/main/pkg/task/inspection/commonlogk8saudit/impl/timelinegrouping_task.go#L202-L255

This is indeed not the role expected for the timeline grouping. Move this logic to another dedicated task.