Clean up flatpak manifest
Remove redundant flags:
https://github.com/flathub/org.gnome.gitg/commit/57260f9317d88610b06e2978419db06226bbdc1f
Remove redundant flags:
https://github.com/flathub/org.gnome.gitg/commit/57260f9317d88610b06e2978419db06226bbdc1f