nixos/treewide: drop dependencies to keys.target

The `keys.target` is used to indicate whether all NixOps keys were
successfully uploaded on an unattended reboot. However this can cause
startup issues e.g. with NixOS containers (see #67265) and can block
boots even though this might not be needed (e.g. with a dovecot2
instance running that doesn't need any of the NixOps keys).

As described in the NixOps manual[1], dependencies to keys should be
defined like this now:

``` nix
{
  systemd.services.myservice = {
    after = [ "secret-key.service" ];
    wants = [ "secret-key.service" ];
  };
}
```

However I'd leave the issue open until it's discussed whether or not to
keep `keys.target` in `nixpkgs`.

[1] https://nixos.org/nixops/manual/#idm140737322342384

nixos/modules/services/networking/softether.nix

@@ -70,8 +70,6 @@ in
 
       systemd.services."softether-init" = {
         description = "SoftEther VPN services initial task";
-        after = [ "keys.target" ];
-        wants = [ "keys.target" ];
         wantedBy = [ "network.target" ];
         serviceConfig = {
           Type = "oneshot";