Revert restrictive validation behavior for DM/WM defaults in the X module

The original idea behind this change (described in ticket #11064) was to
improve the assertions to avoid that users of the X server accidentally
forget to configure a DM or WM.

However this caused several issues with setups that require X, but no DM
or WM. The keymap testcases became instable as well as now disabling DMs
needs to be done explicitly.
(see https://github.com/NixOS/nixpkgs/pull/31268#issuecomment-347080036)

In the end the idea behind the change and #11064 was obviously a
mistake, so reverting it completely for now should be fine.

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

···
   </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>
-  <listitem>
-    <para>
       The option <option>services.logstash.listenAddress</option> is now <literal>127.0.0.1</literal> by default.
       Previously the default behaviour was to listen on all interfaces.
     </para>

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

···
 
       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 == "none" && cfg.session.list != [] then
+          if defaultDM == "" && cfg.session.list != [] then
             (head cfg.session.list).name
           else if any (w: w.name == defaultDM) cfg.session.list then
             defaultDM

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

···
         example = "wmii";
         description = "Default window manager loaded if none have been chosen.";
         apply = defaultWM:
-          if defaultWM == "none" && cfg.session != []  then
-            (head cfg.session).name
-          else if any (w: w.name == defaultWM) cfg.session then
+          if any (w: w.name == defaultWM) cfg.session then
             defaultWM
           else
             throw "Default window manager (${defaultWM}) not found.";

nixos/modules/services/x11/xserver.nix

···
         '';
       };
 
-      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;
···
                 + "${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`. "
-                + "The `default` value looks for enabled WMs/DMs and select the first one.";
-      }
     ];
 
     environment.etc =