uninstall: Use flatpak_dir_list_unused_refs_with_options()

No need to use the FlatpakInstallation version, because we just
needs the refs as strings, not FlatpakInstalledRef:s
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f