[keyserver] Stop persisting primaryIdentityPublicKey from login request

Summary:
We were previously storing the primaryIdentityPublicKey in the public_key column of the cookies table. We're going to strip this logic out before DB migration that removes the public_key column altogether.

Will remove from siweAuthResponder next (and all the helper functions involved).

Test Plan:

1. Login ("traditional")
2. Set breakpoints and observe that public_key column of cookies table isn't being set.
3. Check cookies table make sure that most recent entry lacks public_key.

Reviewers: ashoat, tomek, varun, marcin

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6902