Motivation for this change

Under KDE, launching any graphical application will spit out the
following error as the qemu.desktop file in qemu is invalid:

$ gwenview .
kf5.kservice.services: The desktop entry file "/run/current-system/sw/share/applications/qemu.desktop" has Type= "Application" but no Exec line
kf5.kservice.sycoca: Invalid Service :  "/run/current-system/sw/share/applications/qemu.desktop"

As there are no graphical applications, it's pointless to have this around
in the first place.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.