app: Drop some dead code
app: Drop some dead code

Since commit 096f7d4fb dropped flatpak_dir_check_for_appstream_update(),
this code can never be reached. Drop it to fix coverity issue 1471677.

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak