Remove "Show KRunner" item from desktop context menu

In the past, this was considered to be a way to make KRunner more discoverable by users.

However today Kickoff exposes all the same runners as KRunner, so its full functionality is therefore more discoverable in a different way, so making the KRunner UI itelf more discoverable is not as important.

Furthermore, this was always a questionable way to increase discoverability in the first place, because the menu item did not expose the keyboard shortcut, which is the typical way that people show KRunner. Also, context menus are generally considered shortcuts for experts; novice users don't tend to use context menus very much. But experts are the people more likely to already know about KRunner and its keyboard shortcut, and would never invoke it using the desktop context menu.

Removing this menu item de-clutters the desktop context menu a bit, and we do get complaints that the menu is overwhelming because it has too many things in it. This is a fairly unimportant item that we can remove without any real consequences.

Finally, Marco didn't like the change to refer to KRunner using the actual term "KRunner" in the main UI, so this change removes one of the two places where that happened. :)

Here's what the menu will look like afterwards:

cc @teams/vdg @mart @alex