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.)

pkgs/top-level/metrics.nix

@@ -4,7 +4,8 @@ with pkgs;
 
 runCommand "nixpkgs-metrics"
   { nativeBuildInputs = with pkgs.lib; map getBin [ nix time jq ];
-    requiredSystemFeatures = [ "benchmark" ];
+    #FIXME: the job doesn't work, see issue #76776
+    #requiredSystemFeatures = [ "benchmark" ]; # dedicated machine, by @vcunat last time
   }
   ''
     export NIX_STATE_DIR=$TMPDIR