skypeforlinux: remove nix store path references in desktop files

Because desktop files can be copied to $HOME/.config/autostart, and
eventually the Nix store paths they reference will be garbage collected
and break.

pkgs/applications/networking/instant-messengers/skypeforlinux/default.nix

···
 
     # Fix the desktop link
     substituteInPlace $out/share/applications/skypeforlinux.desktop \
-      --replace /usr/bin/ $out/bin/
+      --replace /usr/bin/ ""
     substituteInPlace $out/share/applications/skypeforlinux-share.desktop \
-      --replace /usr/bin/ $out/bin/
+      --replace /usr/bin/ ""
     substituteInPlace $out/share/kservices5/ServiceMenus/skypeforlinux.desktop \
-      --replace /usr/bin/ $out/bin/
+      --replace /usr/bin/ ""
   '';
 
   meta = with lib; {