[keyserver] Remove payloads from siweAuthResponder ServerErrors

Summary:
Context:
https://phab.comm.dev/D6033#181748
https://phab.comm.dev/D6033#181828

Basically looks like these status payloads are superfluous and aren't used anywhere... so cutting them.

Depends on D6060

Test Plan: Didn't really test... pretty confident this is a benign change.

Reviewers: tomek, ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6061