nixos/wireguard: Remove .path systemd unit for privkey. Fixes #123203

As per `man systemd.path`:

> When a service unit triggered by a path unit terminates
> (regardless whether it exited successfully or failed),
> monitored paths are checked immediately again,
> **and the service accordingly restarted instantly**.

Thus the existence of the path unit made it impossible to stop the
wireguard service using e.g.

    systemctl stop wireguard-wg0.service

Systemd path units are not intended for program inputs such
as private key files.
This commit simply removes this usage; the private key is still
generated by the `generateKeyServiceUnit`.

(cherry picked from commit d344dccf3dc592242f11ef993acb9ecee8d84796)

nixos/modules/services/networking/wireguard.nix

@@ -219,17 +219,6 @@ let
 
   };
 
-  generatePathUnit = name: values:
-    assert (values.privateKey == null);
-    assert (values.privateKeyFile != null);
-    nameValuePair "wireguard-${name}"
-      {
-        description = "WireGuard Tunnel - ${name} - Private Key";
-        requiredBy = [ "wireguard-${name}.service" ];
-        before = [ "wireguard-${name}.service" ];
-        pathConfig.PathExists = values.privateKeyFile;
-      };
-
   generateKeyServiceUnit = name: values:
     assert values.generatePrivateKeyFile;
     nameValuePair "wireguard-${name}-key"
@@ -448,9 +437,6 @@ in
       // (mapAttrs' generateKeyServiceUnit
       (filterAttrs (name: value: value.generatePrivateKeyFile) cfg.interfaces));
 
-    systemd.paths = mapAttrs' generatePathUnit
-      (filterAttrs (name: value: value.privateKeyFile != null) cfg.interfaces);
-
   });
 
 }