[lib] Fix missing usernames
Summary:
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
- We should never do this if usingCSAT is false
- 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
Test Plan: Tested that after this if a new user joins comm ashoat user can see their username.
Reviewers: tomek, ashoat
Reviewed By: tomek
Differential Revision: https://phab.comm.dev/D12674