Refactor to not copy from original method

What does this MR do?

Refactor Knative#set_initial_status to not copy from original method

Instead we can call the original method if the cluster is RBAC. The original method already checks if helm is available, and that it is not not_installable

Existing specs cover this in https://gitlab.com/gitlab-org/gitlab/blob/71c398d03b6ee6993160815528c801319076da83/spec/models/clusters/applications/knative_spec.rb#L26-31

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.