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