flatpak: Reload plugin when a removable remote is unmounted

If a USB drive is unmounted and it contained a loaded USB remote, then
we reload the entire catalog. Ideally this should just remove the USB
category and keywords from the apps in question, but that requires more
book keeping so it should be left for a later stage.
Status Job ID Name Coverage
  Build
passed #365960
build-gnome-software

00:16:11