gsourceclosure: use g_cclosure_marshal_generic

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