core: remove explicit GDestroyNotify cast on g_free / g_object_unref

g_free and g_object_unref are in form of  `void (*)(gpointer)`, which
matches the GDestroyNotify signature. An explicit GDestroyNotify cast on
g_free and g_object_unref is thus not needed.

src/mm-call-list.c

@@ -394,7 +394,7 @@ dispose (GObject *object)
     MMCallList *self = MM_CALL_LIST (object);
 
     g_clear_object (&self->priv->modem);
-    g_list_free_full (self->priv->list, (GDestroyNotify)g_object_unref);
+    g_list_free_full (self->priv->list, g_object_unref);
 
     G_OBJECT_CLASS (mm_call_list_parent_class)->dispose (object);
 }