Group analytics dashboards: Hide header buttons

See thread: #433420 (comment 1695460829)

The group analytics dashboards page does not currently support the Visualization Designer and New dashboard buttons, so we should hide them when viewing the app from a group context.

https://gitlab.com/gitlab-org/gitlab/-/blob/1406ae97c7639cf9bc88aa9f230c0d69bc6eb2dc/ee/app/assets/javascripts/analytics/analytics_dashboards/components/dashboards_list.vue#L175