ustring: Remove useless alternative Stringify ctor
ustring: Remove useless alternative Stringify ctor

This can never be called, because we explicitly specialise Stringify for
const char*, so that specialisation will always be used instead of this.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm