shell: Don’t launch software-properties-gtk when opening repos dialog

This was perhaps a good idea 7 years ago when it was added, but since then we’ve refreshed the repos dialogue, and it supports more different types of packaging. In particular, software-properties-gtk doesn’t support flatpak repos, but we want gnome-software to definitely support that.

Spotted in #2231 (closed).

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