[keyserver] Ignore primaryIdentityPublicKey at login/register/siwe_auth endpoints

Summary: Remove handling of primaryIdentityPublicKey and stop persisting in public_key column of cookies table.

Test Plan: Can still register/login/SIWE as expected. primaryIdentityPublicKey is still accepted in validators so old clients will continue working.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6906