flatpak-utils: don't force OpenURI portal to display its UI

By default, the OpenURI portal will ask the user to select an application to open the link the first few times, but will eventually stop doing so and just open the link automatically. This is what users what, but we're passing the XDP_OPEN_URI_FLAG_ASK flag to suppress this behavior and always show the UI. Since this is annoying users and doesn't have any particular benefit, let's remove it.