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/nsd.nix

@@ -916,9 +916,8 @@ in
     systemd.services.nsd = {
       description = "NSD authoritative only domain name service";
 
-      after = [ "keys.target" "network.target" ];
+      after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
-      wants = [ "keys.target" ];
 
       serviceConfig = {
         ExecStart = "${nsdPkg}/sbin/nsd -d -c ${nsdEnv}/nsd.conf";