StFocusManager: don't unref removed groups

It is not referencing them when adding, and also it is connecting
to the "destroy" signal, emitted on dispose, so there is no risk
of storing finalized objects.

https://bugzilla.gnome.org/show_bug.cgi?id=634781

src/st/st-focus-manager.c

@@ -70,8 +70,7 @@ static void
 st_focus_manager_init (StFocusManager *manager)
 {
   manager->priv = ST_FOCUS_MANAGER_GET_PRIVATE (manager);
-  manager->priv->groups = g_hash_table_new_full (NULL, NULL,
+  manager->priv->groups = g_hash_table_new (NULL, NULL);
-                                                 g_object_unref, NULL);
 }
 
 static gboolean