Switch to default xdg-portal for rfd

The GTK 3 backend of rfd is relatively problematic (it causes
a whole app freeze when the command function isn't async and
a SIGABRT on app exit when it is async: tauri-apps/plugins-workspace#2255)
so use the better-working xdg-portal backend instead (this will
also help with Flatpak compatibility)

This also removes the unused tauri-plugin-dialog dependency
(which conflicts with our rfd features)