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.
