LibGUI: Don't assert in ~GWidgetClassRegistration

These guys are all declared as globals, and their ASSERT_NOT_REACHED
in the destructor doesn't play nice with __cxa_atexit. As in, every
application will assert in __cxa_finalize if this assert isn't removed.

Libraries/LibGUI/GWidget.cpp

@@ -50,7 +50,6 @@ GWidgetClassRegistration::GWidgetClassRegistration(const String& class_name, Fun
 
 GWidgetClassRegistration::~GWidgetClassRegistration()
 {
-    ASSERT_NOT_REACHED();
 }
 
 void GWidgetClassRegistration::for_each(Function<void(const GWidgetClassRegistration&)> callback)