gnome.gnome-tweaks: Drop libsoup dependency

It is not actually used:
https://gitlab.gnome.org/GNOME/gnome-tweaks/-/issues/407

pkgs/desktops/gnome/misc/gnome-tweaks/default.nix

···
 , itstool
 , libhandy
 , libnotify
-, libsoup
 , libxml2
 , pkg-config
 , python3Packages
···
     gtk3
     libhandy
     libnotify
-    libsoup
   ];
 
   pythonPath = with python3Packages; [