sparkleshare: Remove fallback to Flatpak image

The upstream .desktop file has a fallback to flatpack. It is only intended to run if `sparkleshare` isn't in the user's path. But to make matters even worse it will also run if `sparkleshare` exists with a failure code.

Remove this fallback to ensure that users don't download and run unpinned and untrusted code from the internet.

pkgs/applications/version-management/sparkleshare/default.nix

···
   ];
 
   patchPhase = ''
+    # SparkleShare's default desktop file falls back to flatpak.
+    sed -ie "s_^Exec=.*_Exec=$out/bin/sparkleshare_" SparkleShare/Linux/SparkleShare.Autostart.desktop
+
     # Nix will manage the icon cache.
     echo '#!/bin/sh' >scripts/post-install.sh
   '';