This function unregisters font file loader callbacks with the DirectWrite font system. You should implement the font file loader interface by a singleton object. Note that font file loader implementations must not register themselves with DirectWrite inside their constructors and must not unregister themselves in their destructors, because registration and unregistraton operations increment and decrement the object reference count respectively. Instead, registration and unregistration of font file loaders with DirectWrite should be performed outside of the font file loader implementation.