summary: Make flatpak_cache_dir_summary static void
summary: Make flatpak_cache_dir_summary static void

This function always returned NULL, and the return value was never
checked.

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