nixos/displayManager: deprecate separate options for default wm/dm

The upstream session files display managers use have no concept of sessions being composed from
desktop manager and window manager. To be able to set upstream session files as default
session, we need a single option. Having two different ways to set default session would be confusing,
though, so we decided to deprecate the old method.

We also created separate script for each session, just like we already had a separate desktop
file for each one, and started using displayManager.sessionPackages mechanism to make the
session handling more uniform.

nixos/modules/services/x11/desktop-managers/default.nix

@@ -86,23 +86,14 @@ in
       };
 
       default = mkOption {
-        type = types.str;
-        default = "";
+        type = types.nullOr types.str;
+        default = null;
         example = "none";
-        description = "Default desktop manager loaded if none have been chosen.";
-        apply = defaultDM:
-          if defaultDM == "" && cfg.session.list != [] then
-            (head cfg.session.list).name
-          else if any (w: w.name == defaultDM) cfg.session.list then
-            defaultDM
-          else
-            throw ''
-              Default desktop manager (${defaultDM}) not found.
-              Probably you want to change
-                services.xserver.desktopManager.default = "${defaultDM}";
-              to one of
-                ${concatMapStringsSep "\n  " (w: "services.xserver.desktopManager.default = \"${w.name}\";") cfg.session.list}
-            '';
+        description = ''
+          <emphasis role="strong">Deprecated</emphasis>, please use <xref linkend="opt-services.xserver.displayManager.defaultSession"/> instead.
+
+          Default desktop manager loaded if none have been chosen.
+        '';
       };
 
     };