mgr/dashboard: remove Grafana role
Pull request ID:
As it's neither useful nor necessary:
- Useless: roles are meant to control access, but Grafana service is external to dashboard, so there's no way to enforce these policies (any user can access Grafana dashboards independently).
- Unnecessary: Grafana is just a infrastructure component, while roles are meant to control access to Ceph workflows (RBD, RGW, etc.). Hence every Grafana Dashboard should be hidden/displayed based on the specific Ceph workflow where it's embedded in.