Remove "Lock" and "logout" items from default desktop context menu

These items don't have anything to do with the desktop itself, so that's
not really the right place for them, making it unlikely that anyone
would expect to find them in the desktop context menu.

Instead, these items are probably there as a sort of emergency escape
valve so that people can still lock the screen and log out under the
following set of circumstances:

1. They've accidentally removed their launcher menu or primary panel,
   or it has gotten lost due to multi-screen arrangement bugs
2. ...And they don't know how to perform these actions from KRunner
3. ...And they don't know the keyboard shortcuts for these actions

This seems very unlikely, given how we have made it much harder to
accidentally remove your panel and its applets over time, how much
robustness work we've put into multimonitor workflows, and how we now
even have a UI to restore panels that have gotten lost anyway. So
having these actions in the context menu specifically to protect
against such an unlikely happenstance is therefore not worth it, and we
can safely remove them by default (note that users can add them back)
to unclutter the context menu and make it more relevant to the desktop
itself.

---

There is no need to handle the now-orphaned menu separator as QMenu
takes care of automaticlaly suppressing separators when they are the
last item in the menu.

containmentactions/contextmenu/menu.cpp

@@ -75,6 +75,8 @@ void ContextMenu::restore(const KConfigGroup &config)
         disabled.insert(QStringLiteral("configure shortcuts"));
         disabled.insert(QStringLiteral("_run_command"));
         disabled.insert(QStringLiteral("run associated application"));
+        disabled.insert(QStringLiteral("_lock_screen"));
+        disabled.insert(QStringLiteral("_logout"));
     }
     // clang-format on