wireshark: Do $PATH lookup in wireshark.desktop instead of hardcoding derivation

See db236e588d "steam: Do $PATH lookup in steam.desktop [...]".
tl;dr: Otherwise widget/panel/desktop icons in DEs like KDE break.

As upstream's .desktop file does not contain any executable paths,
simply avoid substitution.

Message-Id: <97c3e797-d844-4d0a-9ccf-397745f83aeb@siticom.de>

pkgs/applications/networking/sniffers/wireshark/default.nix

@@ -80,9 +80,6 @@ in stdenv.mkDerivation {
   '' else optionalString withQt ''
     install -Dm644 -t $out/share/applications ../wireshark.desktop
 
-    substituteInPlace $out/share/applications/*.desktop \
-        --replace "Exec=wireshark" "Exec=$out/bin/wireshark"
-
     install -Dm644 ../image/wsicon.svg $out/share/icons/wireshark.svg
     mkdir $dev/include/{epan/{wmem,ftypes,dfilter},wsutil,wiretap} -pv