user-accounts: Don't show_restart_notification when changing Language

Since commit 5e2ed8e7, the user-accounts panel does not present an
option to change the Language for the current user. This should be
done in the "Region & Language" panel instead.

In doing so, the code for launching the restart notification is
never reached in the language_response callback method.

https://bugzilla.gnome.org/show_bug.cgi?id=767065