[lib] Don't reduce KEYSERVER_AUTH_SUCCESS in reduceCurrentUserInfo

Summary:
As discussed on Linear here, the changes in D11140 are not working as desired. The issue is that SET_NEW_SESSION is actually setting the currentUserInfo during keyserver auth, prior to KEYSERVER_AUTH_SUCCESS.

I figure that perhaps the simplest solution here is to remove the code for reducing KEYSERVER_AUTH_SUCCESS from reduceCurrentUserInfo, and to move the error checking to SET_NEW_SESSION.

Test Plan: Make sure that I can still register an account using the new flow with multi-keyserver

Reviewers: inka, tomek

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D11692