nixos/xserver: Remove {desktop,window}Manager.default options

These have been deprecated since NixOS 20.03.

Also fixes evaluation error caused by moving
the `defaultSessionFromLegacyOptions` variable
in 6be2bfcc32d5ee203acf3b85354f5028c8bb50eb

nixos/modules/services/display-managers/default.nix

···
         fi
       '') cfg.sessionPackages}
     '';
-
-  dmDefault = config.services.xserver.desktopManager.default;
-  # fallback default for cases when only default wm is set
-  dmFallbackDefault = if dmDefault != null then dmDefault else "none";
-  wmDefault = config.services.xserver.windowManager.default;
-  defaultSessionFromLegacyOptions = dmFallbackDefault + lib.optionalString (wmDefault != null && wmDefault != "none") "+${wmDefault}";
 in
 {
   options = {
···
                   ${lib.concatStringsSep "\n  " cfg.displayManager.sessionData.sessionNames}
               '';
         };
-        default =
-          if dmDefault != null || wmDefault != null then
-            defaultSessionFromLegacyOptions
-          else
-            null;
-        defaultText = lib.literalMD ''
-          Taken from display manager settings or window manager settings, if either is set.
-        '';
+        default = null;
         example = "gnome";
         description = ''
           Graphical session to pre-select in the session chooser (only effective for GDM, LightDM and SDDM).
···
         '';
       }
     ];
-
-    warnings =
-      lib.mkIf (dmDefault != null || wmDefault != null) [
-        ''
-          The following options are deprecated:
-            ${lib.concatStringsSep "\n  " (map ({c, t}: t) (lib.filter ({c, t}: c != null) [
-            { c = dmDefault; t = "- services.xserver.desktopManager.default"; }
-            { c = wmDefault; t = "- services.xserver.windowManager.default"; }
-            ]))}
-          Please use
-            services.displayManager.defaultSession = "${defaultSessionFromLegacyOptions}";
-          instead.
-        ''
-      ];
 
     # Make xsessions and wayland sessions available in XDG_DATA_DIRS
     # as some programs have behavior that depends on them being present

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

···
         });
       };
 
-      default = mkOption {
-        type = types.nullOr types.str;
-        default = null;
-        example = "none";
-        description = ''
-          **Deprecated**, please use [](#opt-services.displayManager.defaultSession) instead.
-
-          Default desktop manager loaded if none have been chosen.
-        '';
-      };
-
     };
 
   };

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

···
   };
 
   config = {
-    assertions = [
-      {
-        assertion = cfg.desktopManager.default != null || cfg.windowManager.default != null -> cfg.displayManager.defaultSession == defaultSessionFromLegacyOptions;
-        message = "You cannot use both services.displayManager.defaultSession option and legacy options (services.xserver.desktopManager.default and services.xserver.windowManager.default).";
-      }
-    ];
-
     services.displayManager.sessionData.wrapper = xsessionWrapper;
 
     services.xserver.displayManager.xserverBin = "${xorg.xorgserver.out}/bin/X";

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

···
         });
       };
 
-      default = mkOption {
-        type = types.nullOr types.str;
-        default = null;
-        example = "wmii";
-        description = ''
-          **Deprecated**, please use [](#opt-services.displayManager.defaultSession) instead.
-
-          Default window manager loaded if none have been chosen.
-        '';
-      };
-
     };
 
   };