This method will de-register user with current service.
This is usually done when service is no longer necessary.
Note that you won't be able to use this service
anymore. If you wish to use it again, use Register
For example, if you are registered with ICQ gateway and
you want to release that account, you would call this
method. Once this method completes, Jabber server will no
longer connect to ICQ service instead of you.