Drop preferences-desktop-default-applications icon

This icon is superseded by org.xfce.settings.default-applications which is now provided by xfce4-settings (along with the actual preferred applications dialog).

Please note that there are still components that may need updating after this MR is merged, e.g. Thunar (https://gitlab.xfce.org/xfce/thunar/-/blob/master/thunar/thunar-chooser-model.c#L301 and https://gitlab.xfce.org/xfce/thunar/-/blob/master/thunar/thunar-chooser-model.c#L312) or Ristretto (https://gitlab.xfce.org/apps/ristretto/-/blob/master/src/main_window.c#L4265)

I have also updated xfce4-settings (xfce4-settings@0631fb39) already but that change is self-contained and can consequently also be backported to 4.16, the change here should target 4.18.