issues: https://linear.app/comm/issue/ENG-8762/new-users-show-up-as-anonymous-in-chat-with-ashoat#comment-bc006d6a and https://linear.app/comm/issue/ENG-8722/users-were-all-anonymous-after-usingcsat=true-login-on-prod#comment-04b52d64
I am just reverting the changes made in the stack between D12352 and D12514. We don't want to discard the keyserver usernames for now, because

1. We should never do this if usingCSAT is false
2. If usingCSAT is true some of the user might have not migrated yet, so their usernames are not present on identity

Once we implement identity returning usernames from identity-reserved-usernames able as well, we can rever this diff and add a if(usingCSAT) check in discardKeyserverUsernames

Tested that after this if a new user joins comm ashoat user can see their username.