gsourceclosure: use g_cclosure_marshal_generic

For the glib-defined source types, and any source type that defines a
closure callback but not a closure marshal, use
g_cclosure_marshal_generic. And then remove all the other remaining
source closure marshals.

https://bugzilla.gnome.org/show_bug.cgi?id=701511

gio/gcancellable.c

@@ -713,7 +713,6 @@ static GSourceFuncs cancellable_source_funcs =
   cancellable_source_dispatch,
   cancellable_source_finalize,
   (GSourceFunc)cancellable_source_closure_callback,
-  (GSourceDummyMarshal)g_cclosure_marshal_generic,
 };
 
 /**