The static function _g_list_free__g_object_unref0_ is generated if you have a method that returns a list of objects, regardless if anything in that module uses the method.

Test case create two files:
---test.vala---
class Test
{
    public static int main (string[] args)
    {
        var foo = new Foo ();
        foo.get_children ();
        return 0;
    }
}
---foo.vala---
class Foo : Object
{
    public List<Foo> get_children ()
    {
        List<Foo> data = null;
        return data;
    }
}
------------

And compile with -Wall:
$ valac -C test.vala foo.vala 
$ gcc -Wall test.c  foo.c `pkg-config --cflags --libs gobject-2.0` 
foo.c:49:13: warning: ‘_g_list_free__g_object_unref0_’ defined but not used