Broker login: remove identity provider prefix from autogenerated username

Since we have flexible "first broker login" flow for social login and we already handle conflicting username with that, we can likely change the generation of username (in case that UsernameTemplateMapper is not used) and remove the broker prefix from that. The related code is in IdentityBrokerService.authenticated method.

More details in discussion: http://lists.jboss.org/pipermail/keycloak-dev/2016-January/006243.html