Refactor flux uninstall command

- deletes Flux components (deployments and services)
- deletes Flux RBAC (service accounts, cluster roles and cluster role bindings)
- removes the Kubernetes finalizers from Flux custom resources
- deletes Flux custom resource definitions and custom resources
- deletes the namespace where Flux was installed
- preserves the Kubernetes objects and Helm releases that were reconciled on the cluster by Flux

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>