jamiaccount: avoid to block in doUnregister if ERROR_GENERIC

If the account is incorrectly registered (i.e. the common case
is that the DHT socket is not correctly initialized),
setRegistrationState(RegistrationState::ERROR_GENERIC) will be
called and the dht will not be usable.

Calling doUnregister will cause the daemon to block on
dht_->shutdown(). Because the account is not ready to be used,
doUnregister() should do nothing if any error happened during the
registration.

Also, removes 2 unused states.

Change-Id: I0393786afea8c13506c7c87caf265b09182ea0ae