nixos/plasma5: drop enableQt4Support option

Phonon no longer supports Qt4 so this is useless.

nixos/doc/manual/configuration/profiles/graphical.xml

···
  <para>
   It sets <xref linkend="opt-services.xserver.enable"/>,
   <xref linkend="opt-services.xserver.displayManager.sddm.enable"/>,
-  <xref linkend="opt-services.xserver.desktopManager.plasma5.enable"/> (
-  <link linkend="opt-services.xserver.desktopManager.plasma5.enableQt4Support">
-  without Qt4 Support</link>), and
+  <xref linkend="opt-services.xserver.desktopManager.plasma5.enable"/>, and
   <xref linkend="opt-services.xserver.libinput.enable"/> to true. It also
   includes glxinfo and firefox in the system packages list.
  </para>

nixos/doc/manual/release-notes/rl-2003.xml

···
       setting <literal>serviceConfig.PrivateTmp</literal> to <literal>false</literal> for each phpfpm unit.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     KDE’s old multimedia framework Phonon no longer supports Qt 4. For that reason, Plasma desktop also does not have <option>enableQt4Support</option> option any more.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/modules/installer/cd-dvd/installation-cd-graphical-kde.nix

···
   services.xserver = {
     desktopManager.plasma5 = {
       enable = true;
-      enableQt4Support = false;
     };
 
     # Automatically login as nixos.

nixos/modules/profiles/graphical.nix

···
     displayManager.sddm.enable = true;
     desktopManager.plasma5 = {
       enable = true;
-      enableQt4Support = false;
     };
     libinput.enable = true; # for touchpad support on many laptops
   };

nixos/modules/services/x11/desktop-managers/plasma5.nix

···
         example = "vlc";
         description = "Phonon audio backend to install.";
       };
-
-      enableQt4Support = mkOption {
-        type = types.bool;
-        default = true;
-        description = ''
-          Enable support for Qt 4-based applications. Particularly, install a
-          default backend for Phonon.
-        '';
-      };
-
     };
 
   };
 
+  imports = [
+    (mkRemovedOptionModule [ "services" "xserver" "desktopManager" "plasma5" "enableQt4Support" ] "Phonon no longer supports Qt 4.")
+  ];
 
   config = mkMerge [
     (mkIf cfg.enable {
···
 
         # Phonon audio backend
         ++ lib.optional (cfg.phononBackend == "gstreamer") libsForQt5.phonon-backend-gstreamer
-        ++ lib.optional (cfg.phononBackend == "gstreamer" && cfg.enableQt4Support) pkgs.phonon-backend-gstreamer
         ++ lib.optional (cfg.phononBackend == "vlc") libsForQt5.phonon-backend-vlc
-        ++ lib.optional (cfg.phononBackend == "vlc" && cfg.enableQt4Support) pkgs.phonon-backend-vlc
 
         # Optional hardware support features
         ++ lib.optionals config.hardware.bluetooth.enable [ bluedevil bluez-qt ]