Revert "services.xserver: assert that either desktop- or window manager is not "none""

This reverts commit 93c54acf97.
This reopens #30517 @nbp @Ma27

Breaking people's config for this is hardly reasonable as is. If it
absolutely cannot be avoided, at the very least, we need to provide
clear instructions on what people need to upgrade in their config. I
actually had to bisect to the commit, to even find out what property I
should change or define, as the error message was useless. It didn't
even mention a property name.

Discussion on the PR seems to be ongoing, so I'm reverting this, so we
don't break people's systems on unstable.

nixos/doc/manual/release-notes/rl-1803.xml

@@ -90,14 +90,6 @@ following incompatible changes:</para>
       That means that old configuration is not overwritten by default when update to the znc options are made.
     </para>
   </listitem>
-  <listitem>
-    <para>
-      The option <option>services.xserver.desktopManager.default</option> is now <literal>none</literal> by default.
-      An assertion failure is thrown if WM's and DM's default are <literal>none</literal>.
-      To explicitly run a plain X session without and DM or WM, the newly introduced option <option>services.xserver.plainX</option>
-      must be set to true.
-    </para>
-  </listitem>
 </itemizedlist>
 
 </section>

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

@@ -87,8 +87,8 @@ in
 
       default = mkOption {
         type = types.str;
-        default = "none";
-        example = "plasma5";
+        default = "";
+        example = "none";
         description = "Default desktop manager loaded if none have been chosen.";
         apply = defaultDM:
           if defaultDM == "" && cfg.session.list != [] then

nixos/modules/services/x11/xserver.nix

@@ -161,15 +161,6 @@ in
         '';
       };
 
-      plainX = mkOption {
-        type = types.bool;
-        default = false;
-        description = ''
-          Whether the X11 session can be plain (without DM/WM) and
-          the Xsession script will be used as fallback or not.
-        '';
-      };
-
       autorun = mkOption {
         type = types.bool;
         default = true;
@@ -561,9 +552,6 @@ in
                 + "${toString (length primaryHeads)} heads set to primary: "
                 + concatMapStringsSep ", " (x: x.output) primaryHeads;
       })
-      { assertion = cfg.desktopManager.default == "none" && cfg.windowManager.default == "none" -> cfg.plainX;
-        message = "Either the desktop manager or the window manager shouldn't be `none`! To explicitly allow this, you can also set `services.xserver.plainX` to `true`.";
-      }
     ];
 
     environment.etc =