FlatpakDir: use correct function to free FlatpakContext refs
FlatpakDir: use correct function to free FlatpakContext refs

These are not keyfiles. This fixes a crash when launching applications
through gnome-sofware.

GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,amigadave/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,amigadave/flatpak,matthiasclasen/flatpak