nixos/cadvisor: don't enable docker

cadvisor has usecases that don't involve docker, and it's entirely
unexpected for a monitoring service to enable docker.

nixos/modules/services/monitoring/cadvisor.nix

@@ -135,7 +135,6 @@ in {
 
         serviceConfig.TimeoutStartSec=300;
       };
-      virtualisation.docker.enable = mkDefault true;
     })
   ];
 }