gs-plugin-vfuncs: Remove add/remove shortcut and app_set_rating() vfuncs and associated code

No plugin provided support for them. This is functionality that gnome-shell handles; gnome-software shouldn’t be doing it.

All this code was unused with the built-in set of plugins.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Helps: #1472