[keyserver] Stop creating two cookies on siwe registration

Summary:
issue: ENG-8052
We want to match how keyserverAuthResponder was refactored. We want to stop creating two cookies in case of registration. The cookie is created later by processSuccessfulLogin.

Test Plan:
Tested that it is possible to register a user with siwe (ENG-8044 still occurs, but the user is added to the table and receives a user cookie).
Tested that it is possible to login with siwe

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D11919