Remove unused example key from sops secrets specification

nix-system-configs/modules/secrets-config/sops-nix.nix

@@ -25,7 +25,5 @@
     sops.age.keyFile = "/var/lib/sops-nix/key.txt";
     # This will generate a new key if the key specified above does not exist
     sops.age.generateKey = true;
-    # This is the actual specification of the secrets.
-    sops.secrets.example-key = {};
     sops.secrets."songsheet/database" = {};
 }