[keyserver] Remove old current user info

Summary:
ENG-4850
Depends on D9110
We can remove old current user info and simplify the code. Reasoning copied from the task:

1. As you can see in D1463 when OldCurrentUserInfo the version was 86, while our current minimal supported version is 128
2. fetchCurrentUserInfo uses fetchLoggedInUserInfo, which if you look at the code, never actually creates OldCurrentUserInfo structure, it always created the newer type
3. Other code always uses fetchCurrentUserInfo as the source of truth so we can also change their signatures

Test Plan: yarn flow, analyze all changes and make sure that all data is in the end fetched using fetchCurrentUserInfo or from the clients

Reviewers: kamil, inka, atul, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9111