Revert "lib: Allow passing NULL for remote name in update_appstream"
Revert "lib: Allow passing NULL for remote name in update_appstream"

This reverts commit 9a185947a3d3ca5a4005994dcc827c1f9f96e047.

This was problematic wrt how partial errors are reported

flatpak/flatpak,matthiasclasen/xdg-app,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/xdg-app,alexlarsson/xdg-app,amigadave/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/xdg-app,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/xdg-app,alexlarsson/xdg-app,flatpak/flatpak,amigadave/flatpak,alexlarsson/xdg-app,handsome-feng/flatpak,amigadave/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,flatpak/flatpak,alexlarsson/xdg-app,handsome-feng/flatpak,amigadave/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak