You could extend Identity and then override addLoginSuccessfulMessage().
You could also request a feature request...there may already be one in jira.
An alternative is to unset the org.jboss.seam.loginSuccessful key in message.properties:
You can also customize the message if that suits you better. For example, in our application we have a session scoped component called currentUser which contains the user's name after a successful login: