shell: Make sure the "active-panel" property is updated

The (currently unused) cc_shell_get_active_panel() method is useless
unless "active-panel" is actually updated correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=675471

shell/cc-shell.c

@@ -165,7 +165,6 @@ cc_shell_set_active_panel (CcShell *shell,
       if (panel)
         {
           shell->priv->active_panel = g_object_ref (panel);
-          g_object_set (G_OBJECT (panel), "shell", shell, NULL);
         }
       g_object_notify (G_OBJECT (shell), "active-panel");
     }