flatpak (build_manifest): updated org.freedesktop.Platform runtime version: manifests were using an old 1.6 version of the runtime, this patch fixes them by using the current 18.8 one. Thanks to that, the main_loop.patch which was made because of an outdated version of GLib (see https://github.com/flatpak/flatpak/issues/1899) is not needed anymore and has been removed. Some useless extensions have also been removed from default manifest.