Grafana: No phoning home

(No ticket to refer to, this is a small thing I stumbled upon)

Seems Grafana's reporting back home is opt-out: https://github.com/NixOS/nixpkgs/blob/9d1350d9d56411b685ff3de5839ed6728b1bf808/nixos/modules/services/monitoring/grafana.nix#L576-L582

I propose opting out then.

This is what the MR does.