xkb: how can `modifier_map` be completely replaced

When defining a custom keyboard layout with xkb, how can I completely override a modifier? E.g. when I have

xkb_keymap {
    xkb_keycodes  { include "evdev+aliases(qwerty)" };
    xkb_types     { include "complete"      };
    xkb_compat    { include "complete"      };
    xkb_geometry  { include "pc(pc105)"     };
    xkb_symbols   {
        include "pc+us"
        override modifier_map Mod4 { ,  };
    };
};

I expect that Mod4 is only generated for the two "Alt" keys. But

xkbcomp test :0 && xkbcomp :0 generated
grep Mod4 generated

shows that old mappings (from pc105) are still applied

modifier_map Mod4 {  };
    modifier_map Mod4 {  };
    modifier_map Mod4 {  };
    modifier_map Mod4 {  };
    modifier_map Mod4 {  };
    modifier_map Mod4 {  };