nixos/k3s: use default cgroup-driver again

Setting `cgroup-driver=systemd` was originally necessary to match with
docker, else the kubelet would not start (NixOS#111835)

However, since then, docker support has been dropped from k3s (NixOS#177790).
As such, this option is much less necessary.

More importantly, it now seems to be actively causing issues. Due to an
upstream k3s bug, it's resulting in the kubelet and containerd having
different cgroup drivers, which seems to result in some difficult to
debug failure modes.

See
NixOS#181790 (comment)
for a description of this problem.

Removing this flag entirely seems reasonable to me, and it results in
k3s working again on my machine.

nixos/modules/services/cluster/k3s/default.nix

@@ -111,7 +111,6 @@ in
           [
             "${cfg.package}/bin/k3s ${cfg.role}"
           ]
-          ++ (optional (config.systemd.enableUnifiedCgroupHierarchy) "--kubelet-arg=cgroup-driver=systemd")
           ++ (optional cfg.disableAgent "--disable-agent")
           ++ (optional (cfg.serverAddr != "") "--server ${cfg.serverAddr}")
           ++ (optional (cfg.token != "") "--token ${cfg.token}")