metrics: drop requiredSystemFeatures; /cc #76776
metrics: drop requiredSystemFeatures; /cc #76776

The build hasn't worked for months (even on 20.09).  Now also my
dedicated machine broke, and I don't feel like maintaining it until
the job has any chance of succeeding.
(I don't know why exactly it's failing.)

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs