systemd: remove platform assertion

Allow modules to define systemd services on macOS. It won't actually
have any effect, but it would allow modules to define both systemd
services and launchd agents without boilerplate conditionals.

As a consequence of this change, each module would have to check for
compatibility with the OS target instead.

modules/systemd.nix

@@ -10,16 +10,6 @@ let
   mkPathSafeName =
     lib.replaceStrings [ "@" ":" "\\" "[" "]" ] [ "-" "-" "-" "" "" ];
 
-  enabled = cfg.services != { } # \
-    || cfg.slices != { } # \
-    || cfg.sockets != { } # \
-    || cfg.targets != { } # \
-    || cfg.timers != { } # \
-    || cfg.paths != { } # \
-    || cfg.mounts != { } # \
-    || cfg.automounts != { } # \
-    || cfg.sessionVariables != { };
-
   toSystemdIni = lib.generators.toINI {
     listsAsDuplicateKeys = true;
     mkKeyValue = key: value:
@@ -239,28 +229,10 @@ in {
     };
   };
 
-  config = mkMerge [
-    {
-      assertions = [{
-        assertion = enabled -> pkgs.stdenv.isLinux;
-        message = let
-          names = lib.concatStringsSep ", " (lib.attrNames ( # \
-            cfg.services # \
-            // cfg.slices # \
-            // cfg.sockets # \
-            // cfg.targets # \
-            // cfg.timers # \
-            // cfg.paths # \
-            // cfg.mounts # \
-            // cfg.sessionVariables));
-        in "Must use Linux for modules that require systemd: " + names;
-      }];
-    }
-
   # If we run under a Linux system we assume that systemd is
   # available, in particular we assume that systemctl is in PATH.
   # Do not install any user services if username is root.
-    (mkIf (pkgs.stdenv.isLinux && config.home.username != "root") {
+  config = mkIf (pkgs.stdenv.isLinux && config.home.username != "root") {
     xdg.configFile = mkMerge [
       (lib.listToAttrs ((buildServices "service" cfg.services)
         ++ (buildServices "slice" cfg.slices)
@@ -278,8 +250,7 @@ in {
     # running this from the NixOS module then XDG_RUNTIME_DIR is not
     # set and systemd commands will fail. We'll therefore have to
     # set it ourselves in that case.
-      home.activation.reloadSystemd = hm.dag.entryAfter [ "linkGeneration" ]
+    home.activation.reloadSystemd = hm.dag.entryAfter [ "linkGeneration" ] (let
-        (let
       cmd = {
         suggest = ''
           PATH=${dirOf cfg.systemctlPath}:$PATH \
@@ -325,6 +296,5 @@ in {
 
       unset systemdStatus
     '');
-    })
+  };
-  ];
 }