plasma-desktop: remove Discover from default panel

This commit removes shortcuts to KDE Discover from
the default menus of the default Plasma Desktop
installation in NixOS. Right now, in for example
the official NixOS 23.05 Plasma Desktop ISO (as
well as other installation vectors), a broken KDE
Discover link is included in the default toolbar
and other places, leading to a worse user experience,
since NixOS intentionally does not ship the
Plasma Desktop with KDE Discover.

Confirmed working through building an ISO.

pkgs/desktops/plasma-5/plasma-desktop/default.nix

@@ -127,6 +127,7 @@ mkDerivation {
     ./hwclock-path.patch
     ./tzdir.patch
     ./kcm-access.patch
+    ./no-discover-shortcut.patch
   ];
   CXXFLAGS =
     [

pkgs/desktops/plasma-5/plasma-desktop/no-discover-shortcut.patch

@@ -0,0 +1,13 @@
+diff --git a/applets/taskmanager/package/contents/config/main.xml b/applets/taskmanager/package/contents/config/main.xml
+index 6bb27695d..25e621810 100644
+--- a/applets/taskmanager/package/contents/config/main.xml
++++ b/applets/taskmanager/package/contents/config/main.xml
+@@ -85,7 +85,7 @@
+     </entry>
+     <entry name="launchers" type="StringList">
+       <label>The list of launcher tasks on the widget. Usually .desktop file or executable URLs. Special URLs such as preferred://browser that expand to default applications are supported.</label>
+-      <default>applications:systemsettings.desktop,applications:org.kde.discover.desktop,preferred://filemanager,preferred://browser</default>
++      <default>applications:systemsettings.desktop,preferred://filemanager,preferred://browser</default>
+     </entry>
+     <entry name="middleClickAction" type="Enum">
+       <label>What to do on middle-mouse click on a task button.</label>